192 The pancreas  192 The ovaries  192 The testes  192 Summary  194 Test Yourself  194 Websites  194 Answers   • function of carbohydrates, lipids and proteins in  the cell and animals' bodies;   • foods which supply carbohydrates, lipids and  proteins in animal diets.   The elements (simplest chemical substances) found  in an animal's body are all made of basic building  blocks or atoms. The most common elements found  in cells are given in the table below with the symbol  that is used to represent them.   A compound or molecule is formed when two or more atoms join together. Note that some atoms are never found  alone. For example oxygen is always found as mo   alone. For example oxygen is always found as molecules of 2 oxygen atoms (represented as O )   Reactions occur when atoms combine with or separate from other atoms. In the process new products with different  chemical properties are formed.   Chemical reactions can be represented by chemical equations. The starting atoms or compounds are usually put on  the left-hand side of the equation and the products on the right-hand side.   • H 2 + C0 2 gives H 2 C0 3   When some atoms dissolve in water they become charged particles called ions. Some become positively charged  ions and others negatively charged. Ions may have one, two or sometimes three charges.   The table below shows examples of positively and negatively charged ions with the number of their charges.   Positive and negative ions attract one another to hold compounds together.   Ions are important in cells because they conduct electricity when dissolved in water. Substances that ionise in this  way are known as electrolytes.   The molecules in an animal's body fall into two groups: inorganic compounds and organic compounds. The  difference between these is that the first type does not contain carbon and the second type does.   Inorganic compounds include water, sodium chloride, potassium hydroxide and calcium phosphate.   Water is the most abundant inorganic compound, making up over 60% of the volume of cells and over 90% of body  fluids like blood. Many substances dissolve in water and all the chemical reactions that take place in the body do so  when dissolved in water. Other inorganic molecules help keep the acid/base balance ( pH) and concentration of the  blood and other body fluids stable (see Chapter 8).   Organic compounds include carbohydrates, proteins and fats. All organic molecules contain carbon atoms and  they tend to be larger and more complex molecules than inorganic ones. This is largely because each carbon atom  can link with four other atoms. Organic compounds can therefore consist of from one to many thousands of carbon  atoms joined to form chains, branched chains and rings (see diagram below). All organic compounds also contain  hydrogen and they may also contain other elements.   The name "carbohydrate" tells you something about the composition of these "hydrated carbon" compounds. They  contain carbon, hydrogen and oxygen and like water (HO), there are always twice as many hydrogen atoms as  oxygen atoms in each molecule. Carbohydrates are a large and diverse group that includes sugars, starches, glycogen  and cellulose. Carbohydrates in the diet supply an animal with much of its energy and in the animal's body, they  transport and store energy.   Carbohydrates are divided into three major groups based on size: monosaccharides (single sugars), disaccharides  (double sugars) and polysaccharides (multi sugars).   Monosaccharides are the smallest carbohydrate molecules. The most important monosaccharide is glucose which  supplies much of the energy in the cell. It consists of a ring of 6 carbon atoms with oxygen and hydrogen atoms  attached.   Disaccharides are formed when 2 monosaccharides join together. Sucrose (table sugar), maltose, and lactose (milk  sugar), are three important disaccharides. They are broken down to monosaccharides by digestive enzymes in the  gut.   Polysaccharides like starch, glycogen and cellulose are formed by tens or hundreds of monosaccharides linking  together. Unlike mono- and di-saccharides, polysaccharides are not sweet to taste and most do not dissolve in water.   • Starch is the main molecule in which plants store the energy gained from the sun. It is found in grains like  barley and roots like potatoes.   • Glycogen, the polysaccharide used by animals to store energy, is found in the liver and the muscles that move  the skeleton.   • Cellulose forms the rigid cell walls of plants. Its structure is similar to glycogen, but it can't be digested by  mammals. Cows and horses can eat cellulose with the help of bacteria which live in specialised parts of their  gut.   Fats or lipids are important in the plasma membrane around cells and form the insulating fat layer under the skin.  They are also a highly concentrated source of energy, and when eaten in the diet provide twice as much energy per  gram as either carbohydrates or proteins.   Like carbohydrates fats contain carbon, hydrogen and oxygen, but unlike them, there is no particular relationship  between the number of hydrogen and oxygen atoms.   The fats and oils animals eat in their diets are called triglycerides or neutral fats. The building blocks of  triglycerides are 3 fatty acids attached to a backbone of glycerol (glycerine). When fats are eaten the digestive  enzymes break down the molecules into separate fatty acids and glycerol again.   Fatty acids are divided into two kinds: saturated and unsaturated fatty acids depending on how much hydrogen  they contain. The fat found in animals bodies and in dairy products contains mainly saturated fatty acids and tends to  be solid at room temperature. Fish and poultry fats and plant oils contain mostly unsaturated fatty acids and are more  liquid at room temperature.   Phospholipids are lipids that contain a phosphate group. They are important in the plasma membrane of the cell.   Proteins are the third main group of organic compounds in the cell - in fact if you dried out a cell you would find  that about 2/3 of the dry dust you were left with would consist of protein. Like carbohydrates and fats, proteins  contain C, H and O but they all also contain nitrogen. Many also contain sulphur and phosphorus.   In the cell, proteins are an important part of the plasma membrane of the cell, but their most essential role is as  enzymes. These are molecules that act as biological catalysts and are necessary for biochemical reactions to proceed.  Protein is also found as keratin in the skin, feathers and hair, in muscles, as well as in antibodies and some  hormones.   Proteins are built up of long chains of smaller molecules called amino acids. There are 20 common types of amino  acid and different numbers of these arranged in different orders create the multitude of individual proteins that exist  in an animal's body.   Long chains of amino acids often link with other amino acid chains and the resulting protein molecule may twist,  spiral and fold up to make a complex 3-dimensional shape. As an example, see the diagram of the protein lysozyme  below. Believe it or not, this is a small and relatively simple protein.   It is this shape that determines how proteins behave in cells, particularly when they are acting as enzymes. If for any  reason this shape is altered, the protein stops working. This is what happens if proteins are heated or put in a solution  that is too acidic or alkaline. Think what happens to the "white"' of an egg when it is cooked. The "white" contains  the protein albumin, which is changed or "denatured" permanently by cooking. The catastrophic effect that heat has  on enzymes is one of the reasons animals die if exposed to high temperatures.   In the animal's diet, proteins are found in meat (muscle), dairy products, seeds, nuts and legumes like soya. When the  enzymes in the gut digest proteins they break them down into the separate amino acids, which are small enough to be  absorbed into the blood.   • Ions are charged particles, and electrolytes are solutions of ions in water.   • Carbohydrates are made of carbon with hydrogen and oxygen (in the same ratio as water) linked together. The  cell mainly uses carbohydrates for energy.   • Fats are also made of carbon, hydrogen and oxygen. They are a powerful energy source, and are also used for  insulation.   • Proteins are the building materials of the body, and as enzymes make cell reactions happen. They contain  nitrogen as well as carbon, hydrogen and oxygen.   1. What is the difference between an atom and a molecule?   3. Write the equation for carbonic acid splitting into water and carbon dioxide.   4. A solution of table salt in water is an example of an electrolyte.   5. What element is always present in proteins but not usually in fats or carbohydrates?   7. Which will provide you with the most energy — one gram of sugar or one gram of butter?  Test Yourself Answers   1. What is the difference between an atom and a molecule?   An atom is the simplest building block of matter — elements are made of atoms. Molecules are made of atoms  joined together.   2. What is the chemical name for baking soda? And its formula?   3. Write the equation for carbonic acid splitting into water and carbon dioxide.   H2C03 • C02 + H20 4. A solution of table salt in water is an example of an electrolyte. What ions are present in  this solution?   5. What element is always present in proteins but not usually in fats or carbohydrates?   Glucose is a monosaccharide, glycogen is a polysaccharide (and a much bigger molecule); glucose is sweet to  taste and dissolves in water, glycogen is not sweet and is insoluble; glucose is used for energy whereas  glycogen stores energy.   7. Which will provide you with the most energy — one gram of sugar or one gram of butter?   Fats provide approximately 2.5 times more energy than carbohydrates —so the butter wins hands down!   • know that animals belong to the Animal kingdom  and that this is divided into phyla, classes, orders,  families   • know the phylum and class of the more common  animals dealt with in this course   Classification is the process used by scientists to  make sense of the 1 .5 million or so different kinds of  living organisms on the planet. It does this by  describing, naming and placing them in different  groups. As veterinary nurses you are mainly  concerned with the Animal Kingdom but don't forget  that animals rely on the Plant Kingdom for food to  survive. Also many diseases that animals are affected  by are members of the other Kingdoms — fungi,  bacteria and single celled animals.   living organism on Earth ranging from small and simple bacteria to large, complex mammals. From the earliest time  that humans have studied the natural world they have named these living organisms and placed them in different  groups on the basis of their similarities and differences.   Of course we know what a cat, a dog and a whale are but, in some situations using the common names for animals  can be confusing. Problems arise because people in different countries, and even sometimes in the same country,  have different common names for the same animals. For example a cat can be a chat, a Katze, gato, katt, or a  moggie, depending on which language you use. To add to the confusion sometimes the same name is used for  different animals. For example, the name 'gopher' is used for ground squirrels, rodents (pocket gophers), for moles  and in the south-eastern United States for a turtle. This is the reason why all animals have been given an official  scientific or binomial name. Unfortunately these names are always in Latin. For example:   As you can see from the above there are certain rules about writing scientific names:   • The first part is the genus name and is always written with a capital first letter.   • The second name is the species name and is always written in lower case.   The first time you refer to an organism you should write the whole name in full. If you need to keep referring to the  same organism you can then abbreviate the genus name to just the initial. Thus "Canis familiaris" becomes "C.  familiaris" the second and subsequent times you refer to it.   To make some sense of the multitude of living organisms they have been placed in different groups. The method that  has been agreed by biologists for doing this is called the classification system. The system is based on the  assumption that the process of evolution has, over the millennia, brought about slow changes that have converted  simple one-celled organisms to complex multi-celled ones and generated the earth's incredible diversity of life  forms. The classification system attempts to reflect the evolutionary relationships between organisms.   Initially this classification was based only on the appearance of the organism. However, the development of new  techniques has advanced our scientific knowledge. The light microscope and later the electron microscope have  enabled us to view the smallest structures, and now techniques for comparing DNA have begun to clarify still further  the relationships between organisms. In the light of the advances in knowledge the classification has undergone  numerous revisions over time.   At present most biologists divide the living world into 5 kingdoms, namely:   We are concerned here almost entirely with the Animal Kingdom. However, we must not forget that bacteria,  protists, and fungi cause many of the serious diseases that affect animals, and all animals rely either directly or  indirectly on the plant world for their nourishment.   So what are animals? If we were suddenly confronted with an animal we had never seen in our lives before, how  would we know it was not a plant or even a fungus? We all intuitively know part of the answer to this.   • take the food into their bodies and then digest it   If you were tempted to add that animals are furry, run around on four legs and give birth to young that they feed on  milk you were thinking only of mammals and forgetting temporarily that frogs, snakes and crocodiles, birds as well  as fish, are also animals.   These are all members of the group called the vertebrates (or animals with a backbone) and mammals make up only  about 8% of this group. The diagram on the next page shows the percentage of the different kinds of vertebrates.   However, the term animal includes much more than just the Vertebrates. In fact this group makes up only a very  small portion of all animals. Take a look at the diagram below, which shows the size of the different groups of  animals in the Animal Kingdom as proportions of the total number of different animal species. Notice the small size  of the segment representing vertebrates! All the other animals in the Animal Kingdom are animals with no backbone,  or invertebrates. This includes the worms, sea anemones, starfish, snails, crabs, spiders and insects. As more than  90% of the invertebrates are insects, no wonder people worry that insects may take over the world one day!   As we have seen above the Vertebrates are divided into 5 groups or classes namely:   These classes are all based on similarities. For instance all mammals have a similar skeleton, hair on their bodies, are  warm bodied and suckle their young.   Within the subclass containing the true mammals, there are groupings called orders that contain mammals that are  more closely similar or related, than others. Examples of six mammalian orders are given below:   Within each order there are various families. For example within the carnivore mammals are the families:   Even at this point it is possible to find groupings that are more closely related than others. These groups are called  genera (singular genus). For instance within the cat family Felidae is the genus Felis containing the cats, as well as  genera containing panthers, lynxes, and sabre toothed tigers!   The final groups within the system are the species. The definition of a species is a group of animals that can mate  successfully and produce fertile offspring. This means that all domestic cats belong to the species Felis  domesticus, because all breeds of cat whether Siamese, Manx or ordinary House hold cat can cross breed. However,  domestic cats can not mate successfully with lions, tigers or jaguars, so these are placed in separate species, e.g.  Felis leo, Felis tigris and Felis onca.   Even within the same species, there can be animals with quite wide variations in appearance that still breed  successfully. We call these different breeds, races or varieties. For example there are many different breeds of dogs  from Dalmatian to Chihuahua and of cats, from Siamese to Manx and domestic short-hairs, but all can cross breed.  Often these breeds have been produced by selective breeding but varieties can arise in the wild when groups of  animals are separated by a mountain range or sea and have developed different characteristics over long periods of  time.   Families which are divided into  Genera which are divided into  Species.   "Kings Play Cricket On Flat Green Surfaces" OR "Kindly Professors Cannot Often Fail Good Students" are   just two of the phrases students use to remind themselves of the order of these categories - on the other hand you  might like to invent your own.   • The scientific name of an animal has two parts, the genus and the species, and must be written in italics or  underlined.   • The classification system has groupings called phyla, classes, orders, families, genera and species.   • Members within a species can mate and produce fertile offspring.   Work through the exercises in this Classification Worksheet to help you learn how to write scientific names and   la) Re-write this scientific name for an animal correctly: trichosurus Vulpecula.  lb) Can you find out what this animal is?   In fact much more than that. There is an elementary cell biology and classification quiz but the best thing about this  website are the links to tables of characteristics of the different animal groups, for animals both with and without  backbones.   Careful! You could waste all day exploring this wonderful website. Chose an animal or group of animals you want to  know about and you will see not only the classification but photos and details of distribution, behaviour and  conservation status etc.   Nice clear explanation of the different categories used in the classification of animals.   la) Re-write this scientific name for an animal correctly: trichosurus Vulpecula.   Trichosurus vulpecula or Trichosurus vulpecula  lb) Can you find out what this animal is?   No — it is in the order Lagomorpha, not Rodentia. It is a lagomorph!   • the role and function of the plasma membrane;  cytoplasm, ribosomes, rough endoplasmic  reticulum; smooth endoplasmic reticulum,  mitochondria, golgi bodies, lysosomes,  centrioles and the nucleus   • that substances move across the plasma  membrane by passive and active processes   • that passive processes include diffusion,  osmosis and facilitated diffusion and active  processes include active transport, pinocytosis,  phagocytosis and exocytosis   • that mitosis is the means by which ordinary  cells divide   • that meiosis is the process by which the  chromosome number is halved when ova and  sperm are formed   The cell is the basic building block of  living organisms. Bacteria and the  parasite that causes malaria consist of  single cells, while plants and animals  are made up of trillions of cells. Most  cells are spherical or cube shaped but  some are a range of different shapes  (see diagram 3.1).   Most cells are so small that a  microscope is needed to see them,  although a few cells, e.g. the ostrich's  egg, are so large that they could make  a meal for several people.   A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small distances like this are normally expressed  in micrometres or microns (urn). Note there are 1000 urns in every mm).   However, when you use an electron  microscope to increase the  magnification many thousands of times  you see that these seemingly simple  structures are incredibly complex, each  with its own specialized function. For  example the plasma membrane is seen  to be a double layer and the cytoplasm  contains many special structures called  organelles (meaning little organs)  which are described below. A drawing  of the cell as seen with an electron  microscope is shown in diagram 3.3.   Diagram 3.3: An animal cell as seen with an electron microscope   help transport substances from one side of the membrane to the other   Substances need to pass through the membrane to enter or leave the cell and they do so in a number of ways. Some  of these processes require no energy i.e. they are passive, while others require energy i.e. they are active.   Passive processes include: a) diffusion and b) osmosis, while active processes include: c) active transport, d)  phagocytosis, e) pinocytosis and f) exocytosis. These will be described below.   Although you may not know it, you are  already familiar with the process of  diffusion. It is diffusion that causes a  smell (expensive perfume or smelly  socks) in one part of the room to  gradually move through the room so it  can be smelt on the other side.  Diffusion occurs in the air and in  liquids.   Diagram 3.5 shows what happens  when a few crystals of a dark purple  dye called potassium permanganate are  dropped into a beaker of water. The  dye molecules diffuse into the water  moving from high to low concentrations so they become evenly distributed throughout the beaker.   In the body, diffusion causes molecules that are in a high concentration on one side of the cell membrane to move  across the membrane until they are present in equal concentrations on both sides. It takes place because all molecules  have an in-built vibration that causes them to move and collide until they are evenly distributed. It is an absolutely  natural process that requires no added energy.   Small molecules like oxygen, carbon dioxide, water and ammonia as well as fats, diffuse directly through the double  fat layer of the membrane. The small molecules named above as well as a variety of charged particles (ions) also  diffuse through the protein-lined channels. Larger molecules like glucose attach to a carrier molecule that aids their  diffusion through the membrane. This is called facilitated diffusion.   In the animal's body diffusion is important for moving oxygen and carbon dioxide between the lungs and the blood,  for moving digested food molecules from the gut into the blood and for the removal of waste products from the cell.   shows a container divided into two parts by an artificial semi-permeable membrane. Water is poured into one part  while a solution containing salt is poured into the other part. Water can cross the membrane but the salt cannot. The  water crosses the semi-permeable membrane by diffusion until there is an equal amount of water on both sides of the  membrane. The effect of this would be to make the salt solution more diluted and cause the level of the liquid in the  right-hand side of the container to rise so it looked like side B of diagram 3.6. This movement of water across the  semi-permeable membrane is called osmosis. It is a completely natural process that requires no outside energy.   Although it would be difficult to do in practice, imagine that you could now take a plunger and push down on the  fluid in the right-hand side of container B so that it flowed back across the semi-permeable membrane until the level  of fluid on both sides was equal again. If you could measure the pressure required to do this, this would be equal to  the osmotic pressure of the salt solution. (This is a rather advanced concept at this stage but you will meet this term  again when you study fluid balance later in the course).   The plasma membrane of cells acts as  a semi-permeable membrane. If red  blood cells, for example, are placed in  water, the water crosses the membrane  to make the amount of water on both  sides of it equal (see diagram 3.7). This  means that the water moves into the  cell causing it to swell. This can occur  to such an extent that the cell actually  bursts to release its contents. This  bursting of red blood cells is called  haemolysis. In a situation such as this  when the solution on one side of a  semi-permeable membrane has a lower  concentration than that on the other  side, the first solution is said to be  hypotonic to the second.   Diagram 3.7: Osmosis in red cells placed in a hypotonic solution   Now think what would happen if red  blood cells were placed in a salt  solution that has a higher salt  concentration than the solution within  the cells (see diagram 3.8). Such a  bathing solution is called a hypertonic  solution. In this situation the  "concentration" of water within the  cells would be higher than that outside  the cells. Osmosis (diffusion of water)  would then occur from the inside of the  cells to the outside solution, causing  the cells to shrink.   Diagram 3.8: Osmosis in red cells placed in a hypertonic solution   A solution that contains 0.9% salt has  the same concentration as body fluids  and the solution within red cells. Cells  placed in such a solution would neither  swell nor shrink (see diagram 3.9).  This solution is called an isotonic  solution. This strength of salt solution  is often called normal saline and is  used when replacing an animal's body  fluids or when cells like red blood cells  have to be suspended in fluid.   Remember - osmosis is a special kind  of diffusion. It is the diffusion of water  molecules across a semi-permeable  membrane. It is a completely passive  process and requires no energy.   Sometimes it is difficult to remember which way the water molecules move. Although it is not strictly true in a  biological sense, many students use the phrase "SALT SUCKS" to help them remember which way water moves  across the membrane when there are two solutions of different salt concentrations on either side.   As we have seen water moves in and out of the cell by osmosis. All water movement from the intestine into the  blood system and between the blood capillaries and the fluid around the cells (tissue or extra cellular fluid) takes  place by osmosis. Osmosis is also important in the production of concentrated urine by the kidney.   When a substance is transported from a low concentration to a high concentration i.e. uphill against the concentration  gradient, energy has to be used. This is called active transport.   Active transport is important in maintaining different concentrations of the ions sodium and potassium on either side  of the nerve cell membrane. It is also important for removing valuable molecules such as glucose, amino acids and  sodium ions from the urine.   Phagocytosis is sometimes called "cell  eating". It is a process that requires  energy and is used by cells to move  solid particles like bacteria across the  plasma membrane. Finger-like  projections from the plasma membrane  surround the bacteria and engulf them  as shown in diagram 3.10. Once within  the cell, enzymes produced by the  lysosomes of the cell (described later)  destroy the bacteria.   The destruction of bacteria and other  foreign substance by white blood cells  by the process of phagocytosis is a  vital part of the defense mechanisms of the body.   Pinocytosis or "cell drinking" is a very similar process to phagocytosis but is used by cells to move fluids across the  plasma membrane. Most cells carry out pinocytosis (note the pinocytotic vesicle in diagram 3.3).   Exocytosis is the process by means of which substances formed in the cell are moved through the plasma membrane  into the fluid outside the cell (or extra-cellular fluid). It occurs in all cells but is most important in secretory cells  (e.g. cells that produce digestive enzymes) and nerve cells.   Within the plasma membrane is the cytoplasm. It consists of a clear jelly-like fluid called the a) cytosol or  intracellular fluid in which b) cell inclusions, c) organelles and d) microfilaments and microtubules are found.   The cytosol consists mainly of water in which various molecules are dissolved or suspended. These molecules  include proteins, fats and carbohydrates as well as sodium, potassium, calcium and chloride ions. Many of the  reactions that take place in the cell occur in the cytosol.   These are large particles of fat, glycogen and melanin that have been produced by the cell. They are often large  enough to be seen with the light microscope. For example the cells of adipose tissue (as in the insulating fat layer  under the skin) contain fat that takes up most of the cell.   Organelles are the "little organs" of the cell - like the heart, kidney and liver are the organs of the body. They are  structures with characteristic appearances and specific "jobs" in the cell. Most can not be seen with the light  microscope and so it was only when the electron microscope was developed that they were discovered. The main  organelles in the cell are the ribosomes, endoplasmic reticulum, mitochondrion, Golgi complex and lysosomes. A   cell containing these organelles as seen with the electron microscope is shown in diagram 3.3.   Ribosomes are tiny spherical  organelles that make proteins by  joining amino acids together. Many  ribosomes are found free in the  cytosol, while others are attached to  the rough endoplasmic reticulum.   The endoplasmic reticulum (ER) is a  network of membranes that form  channels throughout the cytoplasm  from the nucleus to the plasma  membrane. Various molecules are  made in the ER and transported around  the cell in its channels. There are two types of ER: smooth ER and rough ER.   Smooth ER is where the fats in the cell are made and in some cells, where chemicals like alcohol,  pesticides and carcinogenic molecules are inactivated.   The Rough ER has ribosomes attached to its surface. The function of the Rough ER is therefore to  make proteins that are modified stored and transported by the ER (Diagram 3.11).   mitochondrion) are oval or rod shaped  organelles scattered throughout the  cytoplasm. They consist of two  membranes, the inner one of which is  folded to increase its surface area.  (Diagram 3.12)   Mitochondria are the "power stations" of the cell. They make energy by "burning" food molecules like glucose. This  process is called cellular respiration. The reaction requires oxygen and produces carbon dioxide which is a waste  product. The process is very complex and takes place in a large number of steps but the overall word equation for  cellular respiration is-   Note that cellular respiration is different from respiration or breathing. Breathing is the means by which air is drawn  into and expelled from the lungs. Breathing is necessary to supply the cells with the oxygen required by the  mitochondria and to remove the carbon dioxide produced as a waste product of cellular respiration.   Active cells like muscle, liver, kidney and sperm cells have large numbers of mitochondria.   The Golgi bodies in a cell together  make up the Golgi apparatus. Golgi  bodies are found near the nucleus and  consist of flattened membranes stacked  on top of each other rather like a pile  of plates (see diagram 3.13). The Golgi  apparatus modifies and sorts the  proteins and fats made by the ER, then  surrounds them in a membrane as  vesicles so they can be moved to other  parts of the cell.   Lysosomes are large vesicles that contain digestive enzymes. These break down bacteria and other substances that  are brought into the cell by phagocytosis or pinocytosis. They also digest worn-out or damaged organelles, the  components of which can then be recycled by the cell to make new structures.   Some cells can move and change shape and organelles and chemicals are moved around the cell. Threadlike  structures called microfilaments and microtubules that can contract are responsible for this movement.   These structures also form the projections from the plasma membrane known as flagella (singular flagellum) as in  the sperm tail, and cilia found lining the respiratory tract and used to remove mucus that has trapped dust particles  (see chapter 4).   Microtubules also form the pair of cylindrical structures called centrioles found near the nucleus. These help  organise the spindle used in cell division.   The nucleus is the largest structure in a  cell and can be seen with the light  microscope. It is a spherical or oval  body that contains the chromosomes.  The nucleus controls the development  and activity of the cell. Most cells  contain a nucleus although mature red  blood cells have lost theirs during  development and some muscle cells  have several nuclei.   A double membrane similar in  structure to the plasma membrane  surrounds the nucleus (now called the  nuclear envelope). Pores in this nuclear  membrane allow communication between the nucleus and the cytoplasm.   Within the nucleus one or more spherical bodies of darker material can  be seen, even with the light microscope. These are called nucleoli and  are made of RNA. Their role is to make new ribosomes.   • are passed to a new individual when sex cells fuse together in sexual  reproduction.   In cells that are not dividing the chromosomes are very long and thin and appear as dark grainy material. They  become visible just before a cell divides when they shorten and thicken and can then be counted (see diagram 3.14).   The number of chromosomes in the cells of different species varies but is constant in the cells of any one species  (e.g. horses have 64 chromosomes, cats have 38 and humans 46). Chromosomes occur in pairs (i.e. 32 pairs in the  horse nucleus and 19 in that of the cat). Members of each pair are identical in length and shape and if you look  carefully at diagram 3.15, you may be able to see some of the pairs in the human set of chromosomes.   Cells divide when an animal grows, when  its body repairs an injury and when it  produces sperm and eggs (or ova). There  are two types of cell division: Mitosis and  meiosis.   Mitosis. This is the cell division that  occurs when an animal grows and when  tissues are repaired or replaced. It produces  two new cells (daughter cells) each with a  full set of chromosomes that are identical  to each other and to the parent cell. All the  cells of an animal's body therefore contain  identical DNA.   Meiosis. This is the cell division that  produces the ova and sperm necessary for  sexual reproduction. It only occurs in the  ovary and testis.   The most important function of meiosis it to halve the number of chromosomes so that when the sperm    Diagram 3.16: Division by mitosis results in 2 new cells identical to each other and   fertilises the ovum the normal number  is regained. Body cells with the full set  of chromosomes are called diploid  cells, while gametes (sperm and ova)  with half the chromosomes are called  haploid cells.   Meiosis is a more complex process  than mitosis as it involves two  divisions one after the other and the  four cells produced are all genetically  different from each other and from the  parent cell.   This fact that the cells formed by  meiosis are all genetically different  from each other and from the parent  cell can be seen in litters of kittens  where all the members of the litter are   Diagram 3.17: Division by meiosis results in 4 new cells that are genetically different to   different from each other as well as being different from the parents although they display characteristics of both.   To make the function of the parts of the cell easier to understand and remember you can compare them to a factory.  For example:   The nucleus (1) is the managing director of the factory consulting the blueprint (the chromosomes) (2);   The ER (7) modifies, stores and transports the products around the factory;   The plasma membrane is the factory wall and the gates (8);   • Cells consist of three parts: the plasma membrane, cytoplasm and nucleus.   • Substances pass through the plasma membrane by diffusion (gases, lipids), osmosis (water), active transport  (glucose, ions), phagocytosis (particles), pinocytosis (fluids) and exocytosis (particles and fluids).   • Osmosis is the diffusion of water through a semipermeable membrane. Water diffuses from high water  "concentration" to low water "concentration".   • The cytoplasm consists of cytosol in which are suspended cell inclusions and organelles.   • organelles include ribosomes, endoplasmic reticulum, mitochondria, Golgi bodies and lysosomes.   • The nucleus controls the activity of the cell. It contains the chromosomes that are composed of DNA.   There are several worksheets you can use to help you understand and learn about the cell.   You can then test yourself to see how much you remember.  1. Complete the table below:   2. Red blood cells placed in a 5% salt solution would:   3. Red blood cells placed in a 0.9% solution of salt would be in a:   4. White blood cells remove foreign bodies like bacteria from the body by engulfing them. This process is known as   5. Match the organelle in the left hand column of the table below with its function in the right hand column.   6. The cell division that causes an organism to grow and repairs tissues is called:   7. The cell division that produces sperm and ova is called:   8. TWO important differences between the two types of cell division named by you above are:   Wikipedia is good for almost anything you want to know about cells. Just watch as there is much more here  than you need to know.   The Virtual Cell has beautiful pictures of lots of (virtual?) cell organelles.   Great animations here of diffusion, osmosis, facilitated diffusion, endo- and exocytosis and the development  and action of lysosomes. A bit higher level than you need but still not to be missed.   Diffusion and osmosis. Watch what happens to the water and the solute molecules.   2. Red blood cells placed in a 5% salt solution would shrink as water flowed out of them into the hypertonic salt  solution round them.   3. Red blood cells placed in a 0.9% solution of salt would be in an isotonic solution.   4. White blood cells remove foreign bodies like bacteria from the body by phagocytosis.   6. The cell division that causes an organism to grow and repairs tissues is called mitosis   7. The cell division that produces sperm and ova is called meiosis.   a) In mitosis the number of chromosomes in the cells formed is the same as in the original cell. In meiosis the  number of chromosomes is halved.   b) In mitosis the cells formed are genetically identical to the original cell. In meiosis they are different.   In this chapter, the way the cells of the body are  organised into different tissues is described. You will  find out how these tissues are arranged into organs,  and how the organs form systems such as the  digestive system and the reproductive system. Also  in this chapter, the important concept of homeostasis  is defined. You are also introduced to those pesky  things — directional terms.   • four basic types of tissues, their general function  and where they are found in the body   • the basic organisation of the body of vertebrates  including the main body cavities and the location  of the following major organs: thorax, heart,  lungs, thymus, abdomen, liver, stomach, spleen,  intestines, kidneys, sperm ducts, ovaries, uterus,  cervix, vagina, urinary bladder   directional terms including dorsal, ventral, caudal, cranial, medial, lateral, proximal, distal, rostral, palmar and  plantar. Plus transverse and longitudinal sections   Living organisms move, feed, respire (burn food to make energy), grow, sense their environment, excrete and  reproduce. These seven characteristics are sometimes summarized by the words "MRS GREN". functions of:   Living organisms are made from cells which are organised into tissues and these are themselves combined to form  organs and systems.   Skin cells, muscle cells, skeleton cells and nerve cells, for example. These different types of cells are not just  scattered around randomly but similar cells that perform the same function are arranged in groups. These collections  of similar cells are known as tissues.   There are four main types of tissues in animals. These are:   Epithelium (plural epithelia) is tissue that covers and lines. It covers an organ or lines a tube or space in the body.  There are several different types of epithelium, distinguished by the different shapes of the cells and whether they  consist of only a single layer of cells or several layers of cells.   Squamous epithelium consists of a single layer of flattened cells that  are shaped rather like 'crazy paving'. It is found lining the heart, blood  vessels, lung alveoli and body cavities (see diagram 4.1). Its thinness  allows molecules to diffuse across readily.   Cuboidal epithelium consists of a single layer of cube shaped cells. It is  rare in the body but is found lining kidney tubules (see diagram 4.2).  Molecules pass across it by diffusion, osmosis and active transport.   Columnar epithelium consists of column shaped cells. It is found lining  the gut from the stomach to the anus (see diagram 4.3). Digested food  products move across it into the blood stream.   Columnar epithelium with cilia on the free surface (also known as the  apical side of the cell) lines the respiratory tract, fallopian tubes and  uterus (see diagram 4.4). The cilia beat rhythmically to transport  particles.   The cells in transitional epithelium can move over one another  allowing it to stretch. It is found in the wall of the bladder (see diagram  4.5).   Epithelia with several layers of cells are found where toughness and  resistance to abrasion are needed.   Stratified squamous epithelium has many layers of flattened cells. It is   found lining the mouth, cervix and vagina. Cells at the base divide and   push up the cells above them and cells at the top are worn or pushed off the surface (see diagram 4.6). This type of   Keratinised stratified squamous epithelium has a tough waterproof protein called keratin deposited in the cells. It  forms the skin found covering the outer surface of mammals. (Skin will be described in more detail in Chapter 5).   Blood, bone, tendons, cartilage, fibrous connective tissue and fat (adipose) tissue are all classed as connective  tissues. They are tissues that are used for supporting the body or transporting substances around the body. They also  consist of three parts: they all have cells suspended in a ground substance or matrix and most have fibres running  through it.   Blood consists of a matrix - plasma, with several types of cells and cell fragments suspended in it. The fibres are  only evident in blood that has clotted. Blood will be described in detail in chapter 8.   Lymph is similar in composition to blood plasma with various types of white blood cell floating in it. It flows in  lymphatic vessels.   Connective tissue 'proper' consists of a jelly-like matrix with a dense  network of collagen and elastic fibres and various cells embedded in it.  There are various different forms of 'proper' connective tissue (see 1, 2  and 3 below).   Loose connective tissue is a sticky whitish substance that fills the  spaces between organs. It is found in the dermis of the skin (see diagram 4.7).   Dense connective tissue contains lots of thick fibres and is very strong. It forms tendons, ligaments and heart valves  and covers bones and organs like the kidney and liver.   Adipose tissue consists of cells filled with fat. It forms the fatty layer under the dermis of the skin, around the  kidneys and heart and the yellow marrow of the bones.   Cartilage is the 'gristle' of the meat. It consists of a tough jelly-like  matrix with cells suspended in it. It may contain collagen and elastic  fibres. It is a flexible but tough tissue and is found at the ends of bones,  in the nose, ear and trachea and between the vertebrae (see diagram  4.8).   Bone consists of a solid matrix made of calcium salts that give it its hardness. Collagen fibres running through it  give it its strength. Bone cells are found in spaces in the matrix. Two types of bone are found in the skeleton namely  spongy and compact bone. They differ in the way the cells and matrix are arranged. (See Chapter 6 for more details  of bone).   Muscle tissue is composed of cells that contract and move the body. There are three types of muscle tissue:   Smooth muscle consists of long and slender cells with a central nucleus  (see diagram 4.9). It is found in the walls of blood vessels, airways to  the lungs and the gut. It changes the size of the blood vessels and helps  move food and fluid along. Contraction of smooth muscle fibres occurs  without the conscious control of the animal.   muscle) has striped fibres with alternating light and dark bands. It is  attached to bones and is under the voluntary control of the animal (see  diagram 4.10).   Cardiac muscle is found only in the walls of the heart where it  produces the 'heart beat'. Cardiac muscle cells are branched cylinders  with central nuclei and faint stripes (see diagram 4.11). Each fibre  contracts automatically but the heart beat as a whole is controlled by  the pacemaker and the involuntary autonomic nervous system.   Nervous tissue forms the nerves, spinal cord and brain. Nerve cells or  neurons consist of a cell body and a long thread or axon that carries  the nerve impulse. An insulating sheath of fatty material (myelin)  usually surrounds the axon. Diagram 4.12 shows a typical motor  neuron that sends messages to muscles to contract.   We are so familiar with animals with backbones (i.e. vertebrates) that it seems rather unnecessary to point out that  the body is divided into three sections. There is a well-defined head that contains the brain, the major sense organs  and the mouth, a trunk that contains the other organs and a well-developed tail. Other features of vertebrates may be  less apparent. For instance, vertebrates that live on the land have developed a flexible neck that is absent in fish  where it would be in the way of the gills and interfere with streamlining. Mammals but not other vertebrates have a  sheet of muscle called the diaphragm that divides the trunk into the chest region or thorax and the abdomen.   In contrast to many primitive animals, vertebrates have spaces or body  cavities that contain the body organs. Most vertebrates have a single  body cavity but in mammals the diaphragm divides the main cavity  into a thoracic and an abdominal cavity. In the thoracic cavity the  heart and lungs are surrounded by their own membranes so that  cavities are created around the heart - the pericardial cavity, and  around the lungs — the pleural cavity (see diagram 4.13).   Just as the various parts of the cell work together to perform the cell's  functions and a large number of similar cells make up a tissue, so many  different tissues can "cooperate" to form an organ that performs a  particular function. For example, connective tissues, epithelial tissues,  muscle tissue and nervous tissue combine to make the organ that we  call the stomach. In turn the stomach combines with other organs like  the intestines, liver and pancreas to form the digestive system (see  diagram 4.14).   At this point it would be a good idea to make yourself familiar with the  major organs and their positions in the body of a mammal like the  rabbit. Diagram 4.15 shows the main body organs.   Organs do not work in isolation but function in cooperation with other  organs and body structures to bring about the MRS GREN functions  necessary to keep an animal alive. For example the stomach can only  work in conjunction with the mouth and oesophagus (gullet). These  provide it with the food it breaks down and digests. It then needs to  pass the food on to the intestines etc. for further digestion and  absorption. The organs involved with the taking of food into the body,  the digestion and absorption of the food and elimination of waste  products are collectively known as the digestive system.   The skin covering the body consists of two layers, the epidermis and dermis. Associated with these layers are  hairs, feathers, claws, hoofs, glands and sense organs of the skin.   This can be divided into the bones of the skeleton and the joints where the bones move over each other.   The muscles, in conjunction with the skeleton and joints, give the body the ability to move.   This is also known as the circulatory system. It consists of the heart, the blood vessels and the blood. It  transports substances around the body.   This system is responsible for collecting and "cleaning" the fluid that leaks out of the blood vessels. This fluid  is then returned to the blood system. The lymphatic system also makes antibodies that protect the body from  invasion by bacteria etc. It consists of lymphatic vessels, lymph nodes, the spleen and thymus glands.   This is the system involved with bringing oxygen in the air into the body and getting rid of carbon dioxide,  which is a waste product of processes that occur in the cell. It is made up of the trachea, bronchi, bronchioles,  lungs, diaphragm, ribs and muscles that move the ribs in breathing.   This is also known as the gastrointestinal system, alimentary system or gut. It consists of the digestive tube  and glands like the liver and pancreas that produce digestive secretions. It is concerned with breaking down  the large molecules in foods into smaller ones that can be absorbed into the blood and lymph. Waste material  is also eliminated by the digestive system.   This is also known as the renal system. It removes waste products from the blood and is made up of the  kidneys, ureters and bladder.   This is the system that keeps the species going by making new individuals. It is made up of the ovaries, uterus,  vagina and fallopian tubes in the female and the testes with associated glands and ducts in the male.   This system coordinates the activities of the body and responses to the environment. It consists of the sense  organs (eye, ear, semicircular canals, and organs of taste and smell), the nerves, brain and spinal cord.   This is the system that produces chemical messengers or hormones. It consists of various endocrine glands  (ductless glands) that include the pituitary, adrenal, thyroid and pineal glands as well as the testes and ovary.   All the body systems, except the reproductive system, are involved with keeping the conditions inside the animal  more or less stable. This is called homeostasis. These constant conditions are essential for the survival and proper  functioning of the cells, tissues and organs of the body. The skin, for example, has an important role in keeping the  temperature of the body constant. The kidneys keep the concentration of salts in the blood within limits and the islets  of Langerhans in the pancreas maintain the correct level of glucose in the blood through the hormone insulin. As  long as the various body processes remain within normal limits, the body functions properly and is healthy. Once  homeostasis is disturbed disease or death may result. (See Chapters 12 andl6 for more on homeostasis).   In the following chapters the systems  of the body in the list above will be  covered one by one. For each one the  structure of the organs involved will be  described and the way they function  will be explained.   In order to describe structures in the  body of an animal it is necessary to  have a system for describing the  position of parts of the body in relation  to other parts. For example it may be  necessary to describe the position of  the liver in relation to the diaphragm,  or the heart in relation to the lungs.  Certainly if you work further with  animals, in a veterinary clinic for  example, it will be necessary to be able  to accurately describe the position of  an injury. The terms used for this are  called directional terms.   The most common directional terms are right and left. However, even these are not completely straightforward  especially when looking at diagrams of   animals. The convention is to show the left side of the animal or organ  on the right side of the page. This is the view you would get looking  down on an animal lying on its back during surgery or in a  post-mortem. Sometimes it is useful to imagine 'getting inside' the  animal (so to speak) to check which side is which. The other common  and useful directional terms are listed below and shown in diagram  4.16.    Closer to the body than (only used for structures on limbs)    There are more grey hairs in the rostral part of the head   Note that we don't use the terms superior and inferior for animals. They are only used to describe the position of  structures in the human body (and possibly apes) where the upright posture means some structures are above or  superior to others.   In order to look at the structure of some of the parts or organs of the body it may be necessary to cut them open or  even make thin slices of them that they can be examined under the microscope. The direction and position of slices  or sections through an animal's body have their own terminology.   If an animal or organ is sliced lengthwise this section is called a longitudinal or sagittal section. This is sometimes  abbreviated to LS.   If the section is sliced crosswise it is called a transverse or cross section. This is sometimes abbreviated to TS or  XS (see diagram 4.17).   • The characteristics of living organisms can be summarised by the words "MRS GREN."   • There are 4 main types of tissue namely: epithelial, connective, muscle and nervous tissues.   • Epithelial tissues form the skin and line the gut, respiratory tract, bladder etc.   • Connective tissues form tendons, ligaments, adipose tissue, blood, cartilage and bone, and are found in the  dermis of the skin.   • Muscular tissues contract and consist of 3 types: smooth, skeletal and cardiac.   • Vertebrate bodies have a head, trunk and tail. Body organs are located in body cavities. 11 body systems  perform essential body functions most of which maintain a stable environment or homeostasis within the animal.   • Directional terms describe the location of parts of the body in relation to other parts.   Students often find it hard learning how to use directional terms correctly. There are two worksheets to help you with  these and another on tissues.   1. Living organisms can be distinguished from non-living matter because they usually move and grow. Name 5 other  functions of living organisms:   3. Name the body cavity in which the following organs are found:   a) The stomach is cranial to the diaphragm - true I false   c) The spleen is roughly the same size as the stomach and lies near it - true I false   d) The small intestine is proximal to the kidneys - true I false   A fabulous site showing you actual photos of microscope sections of the different tissues with short explanations of  each and even quizzes.   Overview of the different organ systems (in humans) and their functions in maintaining homeostasis in the body.  http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookANIMORGSYS.html   Directional terms for animals. A little more detail than required but still great.  http://en.wikipedia.org/wiki/Anatomical_terms_of_location   1. Living organisms can be distinguished from non-living matter because they usually move and grow. They also  excrete, respire, are sensitive to the environment, reproduce and feed. 2. What tissue types would you find:   d) Blood (a type of connective tissue) flows through blood vessels.   3. Name the body cavity in which the following organs are found:   a) The heart is located in the pericardial cavity (situated in the thoracic cavity).   d) The lungs are situated in the pleural cavity (also situated in the thoracic cavity).   d) The circulatory system includes the heart, blood vessels and blood.   5. Homeostasis is the maintenance of constant conditions in an animal's body.   a) The stomach is cranial to the diaphragm -false. It is caudal to the diaphragm.   b) The heart lies in the pelvic cavity - false. It lies in the thoracic cavity or to be more precise in the pleural  cavity.   c) The spleen is roughly the same size as the stomach and lies near it - true   d) The small intestine is proximal to the kidneys - false (remember the term proximal is only used when  describing the relationship of parts of the limbs to each other). Perhaps it would be better to say that the small  intestine is ventral to the kidneys.   f) The liver is cranial to the heart - false. The liver is caudal to the heart.   The skin is the first of the eleven body systems to be  described. Each chapter from now on will cover one  body system.   System is, in fact, the largest organ of the body. It  has a complex structure, being composed of many  different tissues. It performs many functions that are  important in maintaining homeostasis in the body.  Probably the most important of these functions is the  control of body temperature. The skin also protects  the body from physical damage and bacterial  invasion. The skin has an array of sense organs  which sense the external environment, and also cells  which can make vitamin D in sunlight.   The skin is one of the first systems affected when an  animal becomes sick so it is important for anyone  working with animals to have a sound knowledge of  the structure and functioning of the skin so they can  quickly recognise signs of disease.   the structure and function of keratin skin structures including calluses, scales, nails, claws, hoofs and horns   that antlers are not made either of keratin or in the epidermis   the structure of the different types of feathers and the function of preening   the general structure and function of sweat, scent, preen and mammary glands   the basic functions of the skin in sensing stimuli, temperature control and production of vitamin D   Skin comes in all kinds of textures and forms. There is the dry warty skin of toads and crocodiles, the wet slimy skin  of fish and frogs, the hard shell of tortoises and the soft supple skin of snakes and humans. Mammalian skin is  covered with hair, that of birds with feathers, and fish and reptiles have scales. Pigment in the skin, hairs or feathers  can make the outer surface almost any colour of the rainbow.   As humans, it is often the skin of an animal that gives it its appeal to us or repels us. We love the soft feel of a cat's  coat but perhaps can't bear to touch a snake. As the main part of an animal visible to us, the skin can often give us  clues to the health of an animal. A healthy animal will have a clean, glowing, flexible skin, while ill health may show  itself as an abnormal colour or texture.   Skin is one of the largest organs of the body, making up 6-8% of the total body weight. It consists of two distinct  layers. The top layer is called the epidermis and under that is the dermis (see diagram 5.1).   The epidermis is the layer that bubbles up when we have a blister and as we know from this experience, it has no  blood or nerves in it. The cells at the base of the epidermis continually divide and push the cells above them  upwards. As these cells move up they die and become the dry flaky scales that fall off the skin surface. The cells in  the epidermis die because a special protein called keratin is deposited in them. Keratin is an extremely important  substance for it makes the skin waterproof. Without it land vertebrates like reptiles, birds and mammals would, like  frogs, be able to survive only in damp places.   Reptiles, birds and mammals all have nails or claws on the ends of their toes. They protect the end of the toe and  may be used for grasping, grooming, digging or in defense. They are continually worn away and grow continuously  from a growth layer at their base (see diagram 5.2).   Hoofs are found in sheep, cows, horses etc. otherwise known as ungulate mammals. These are animals that have  lost toes in the process of evolution and walk on the "nails" of the remaining toes. The hoof is a cylinder of horny  material that surrounds and protects the tip of the toe (see diagram 5.3).   True horns are made of keratin and are found in sheep, goats and cattle. They are never branched and, once grown,  are never shed. They consist of a core of bone arising in the dermis of the skin and are fused with the skull. The horn  itself forms as a hollow cone-shaped sheath around the bone (see diagram 5.4).   The antlers of male deer have quite a different structure. They are not formed in the epidermis and do not consist of  keratin but are entirely of bone. They are shed each year and are often branched, especially in older animals. When  growing they are covered in skin called velvet that forms the bone. Later the velvet is shed to leave the bony antler.  The velvet is often removed artificially to be sold in Asia as a traditional medicine (see diagram 5.5).   Other animals have projections on their heads that are not true horns either. The horns on the head of giraffes are  made of bone covered with skin and hair, and the 'horn' of a rhinoceros is made of modified and fused hair-like  structures.   Hair is also made of keratin and develops in the epidermis. It covers the body of most mammals where it acts as an  insulator and helps to regulate the temperature of the body (see below). The colour in hairs is formed from the same  pigment, melanin that colours the skin. Coat colour may help camouflage animals and sometimes acts to attract the  opposite sex.   Hairs lie in a follicle and grow from a root that is well supplied with blood vessels. The hair itself consists of layers  of dead keratin - containing cells and usually lies at a slant in the skin. A small bundle of smooth muscle fibres (the  hair erector muscle) is attached to the side of each hair and when this contracts the hair stands on end. This  increases the insulating power of the coat and is also used by some animals to make them seem larger when  confronted by a foe or a competitor(see diagram 5.6).   The whiskers of cats and the spines of hedgehogs are examples of special types of hairs.   The lightness and stiffness of keratin is also a key to bird flight. In the form of feathers it provides the large airfoils  necessary for flapping and gliding flight. In another form, the light fluffy down feathers, also made of keratin, are  some of the best natural insulators known. This superior insulation is necessary to help maintain the high body  temperatures of birds.   Countour feathers are large feathers that cover the body, wings and tail. They have an expanded vane that provides  the smooth, continuous surface that is required for effective flight. This surface is formed by barbs that extend out  from the central shaft. If you look carefully at a feather you can see that on either side of each barb are thousands of  barbules that lock together by a complex system of hooks and notches, if this arrangement becomes disrupted, the  bird uses its beak to draw the barbs and barbules together again in an action known as preening (see diagram 5.7).   Down feathers are the only feathers covering a chick and form the main insulation layer under the contour feathers of  the adult. They have no shaft but consist of a spray of simple, slender branches (see diagram 5.8).   Pin feathers have a slender hair-like shaft often with a tiny tuft of barbs on the end. They are found between the other  feathers and help tell a bird how its feathers are lying (see diagram 5.9).   Glands are organs that produce and secrete fluids. They are usually divided into two groups depending upon whether  or not they have channels or ducts to carry their products away. Glands with ducts are called exocrine glands and  include the glands found in the skin as well as the glands that produce digestive enzymes in the gut. Endocrine  glands have no ducts and release their products (hormones) directly into the blood stream. The pituitary and adrenal  glands are examples of endocrine glands.   Most vertebrates have exocrine glands in the skin that produce a variety of secretions. The slime on the skin of fish  and frogs is mucus produced by skin glands and some fish and frogs also produce poison from modified glands. In  fact the skin glands of some frogs produce the most poisonous chemicals known. Reptiles and birds have a dry skin  with few glands. The preen gland, situated near the base of the bird's tail, produces oil to help keep the feathers in  good condition. Mammals have an array of different skin glands. These include the wax producing, sweat, sebaceous  and mammary glands.   Sebaceous glands secrete an oily secretion into the hair follicle. This secretion, known as sebum, keeps the hair  supple and helps prevent the growth of bacteria (see diagram 5.6).   Sweat glands consist of a coiled tube and a duct leading onto the skin surface. Their appearance when examined  under the microscope inspired one of the first scientists to observe them to call them "fairies' intestines" (see diagram  5.1). Sweat contains salt and waste products like urea and the evaporation of sweat on the skin surface is one of the  major mechanisms for cooling the body of many mammals. Horses can sweat up to 30 litres of fluid a day during  active exercise, but cats and dogs have few sweat glands and must cool themselves by panting. Scent in the sweat of  many animals is used to mark territory or attract the opposite sex.   Mammary glands are only present in mammals. They are thought to be modified sebaceous glands and are present  in both sexes but are rarely active in males (see diagram 5.10). The number of glands varies from species to species.  They open to the surface in well-developed nipples. Milk contains proteins, sugars, fats and salts, although the exact  composition varies from one species to another.   A moderate amount of UV in sunlight is necessary for the skin to form vitamin D. This vitamin prevents bone  disorders like rickets to which animals reared indoors are susceptible. Excessive exposure to the UV in sunlight can  be damaging and the pigment melanin, deposited in cells at the base of the epidermis, helps to protect the underlying  layers of the skin from this damage. Melanin also colours the skin and variations in the amount of melanin produces  colours from pale yellow to black.   Excess exposure to the sun can cause sunburn. This is common in humans, but light skinned animals like cats and  pigs can also be sunburned, especially on the ears. Skin cancer can also result from excessive exposure to the sun. As  holes in the ozone layer increase exposure to the sun's UV rays, so too does the rate of skin cancer in humans and  animals.   The underlying layer of the skin, known as the dermis, is much thicker but much more uniform in structure than the  epidermis (see diagram 5.1). It is composed of loose connective tissue with a felted mass of collagen and elastic  fibres. It is this part of the skin of cattle and pigs etc. that becomes commercial leather when treated,. The dermis is  well supplied with blood vessels, so cuts and burns that penetrate down into the dermis will bleed or cause serious  fluid loss. There are also numerous nerve endings and touch receptors in the dermis because, of course, the skin is  sensitive to touch, pain and temperature.   When looking at a section of the skin under the microscope you can see hair follicles, sweat and sebaceous glands  dipping down into the dermis. However, these structures do not originate in the dermis but are derived from the  epidermis.   In the lower levels of the dermis is a layer of fat or adipose tissue (see diagram 5.1). This acts as an energy store and  is an excellent insulator especially in mammals like whales with little hair.   Vertebrates can be divided into two groups depending on whether or not they control their internal temperature.  Amphibia (frogs) and reptiles are said to be'cold blooded" (poikilothermic) because their body temperature  approximately follows that of the environment. Birds and mammals are said to be warm blooded (homoiothermic)  because they can maintain a roughly constant body temperature despite changes in the temperature of the  environment.   Heat is produced by the biochemical reactions of the body (especially in the liver) and by muscle contraction. Most  of the heat lost from the body occurs via the skin. It is therefore not surprising that many of the mechanisms for  controlling the temperature of the body operate here.   When an animal is in a cold environment and needs to reduce heat loss the erector muscles contract causing the hair  or feathers to rise up and increase the layer of insulating air trapped by them (see diagram 5.11a).   Heat loss from the skin surface can also be reduced by the contraction of the abundant blood vessels that lie in the  dermis. This takes blood flow to deeper levels, so reducing heat loss and causing pale skin (see diagram 5.12a).   Shivering caused by twitching muscles produces heat that also helps raise the body temperature.   There are two main mechanisms used by animals to increase the amount of heat lost from the skin when they are in a  hot environment or high levels of activity are increasing internal heat production. The first is the expansion of the  blood vessels in the dermis so blood flows near the skin surface and heat loss to the environment can take place. The  second is by the production of sweat from the sweat glands (see diagram 5.12b). The evaporation of this liquid on  the skin surface produces a cooling effect.   The mechanisms for regulating body temperature are under the control of a small region of the brain called the  hypothalamus. This acts like a thermostat.   The amount of heat that can be lost from the surface of the body is related to the area of skin an animal has in  relation to the total volume of its body.   Small animals like mice have a very large skin area compared to their total volume. This means they tend to loose  large amounts of heat and have difficulty keeping warm in cold weather. They may need to keep active just to  maintain their body temperature or may hibernate to avoid the problem.   Large animals like elephants have the opposite problem. They have only a relatively small skin area in relation to  their total volume and may have trouble keeping cool. This is one reason that these large animals tend to have sparse  coverings of hair.   • Skin consists of two layers: the thin epidermis and under it the thicker dermis.   • The Epidermis is formed by the division of base cells that push those above them towards the surface where they  die and are shed.   • Keratin, a protein, is deposited in the epidermal cells. It makes skin waterproof.   • Various skin structures formed in the epidermis are made of keratin. These include: claws, nails, hoofs, horn, hair  and feathers.   • Various Exocrine Glands (with ducts) formed in the epidermis include sweat, sebaceous, and mammary glands.   • Melanin deposited in cells at the base of the epidermis protects deeper cells from the harmful effects of the sun.   • The Dermis is composed of loose connective tissue and is well supplied with blood.   • Body Temperature is controlled by: sweat, hair erection, dilation and contraction of dermal capillaries and  shivering.   Use the Skin Worksheet to help you learn all about the skin.   Now use this Skin Test Yourself to see how much you have learned and remember.   1 . The two layers that form the skin are the a) and b)   2. The special protein deposited in epidermal cells to make them waterproof is:  keratin   3. Many important skin structures are made of keratin. These include:  hair,nails,foot pads,feathers, scales on reptiles   4. Sweat, sebaceous and mammary glands all have ducts to the outside. These kind of glands are known as:  exocrine   5. What is the pigment deposited in skin cells that protects underlying skin layers from the harmful effects of the  sun?   6. How does the skin help cool an animal down when it is active or in a hot environment?  Panting and secretion from sweat glands.   7. Name two mechanisms by means of which the skin helps prevent heat loss when an animal is in a cold  environment.   • http://www.auburn.edu/academic/classes/zy/0301/Topic6/Topic6.html Comparative anatomy  Good on keratin skin structures - hairs, feathers, horns etc.   • http://www.earthlife.net/birds/feathers.html The wonder of bird feathers  Fantastic article on bird feathers with great pictures.   Wikipedia on (human) skin. Good as usual, but more information than you need.   1 . The two layers that form the skin are the: epidermis and dermis   2. The special protein deposited in epidermal cells to make them waterproof is keratin   3. Skin structures made of keratin include scales, beaks, claws, nails, hoofs, hairs and feathers.   4. Sweat, sebaceous and mammary glands are all exocrine glands. Meaning they all have ducts through which they  expel their products to the outside.   5. The pigment deposited in skin cells that protects underlying skin layers from the harmful effects of the sun is  melanin.   6. The skin helps cool an animal down when it is active or in a hot environment by producing sweat and dilating the  capillaries in the dermis   7. The skin helps prevent heat loss when an animal is in a cold environment by:   a) contraction of the hair erector muscles to make the hairs stand up and trap an insulating layer of air.   b) constriction of the capillaries in the dermis to reduce heat loss through the body surface.   • the basic structure of a vertebrae and the regions  of the vertebral column   • the main bones of the fore and hind limbs, and  their girdles and be able to identify them in a live  cat, dog, or rabbit   Fish, frogs, reptiles, birds and mammals are called  vertebrates, a name that comes from the bony  column of vertebrae (the spine) that supports the  body and head. The rest of the skeleton of all these  animals (except the fish) also has the same basic  design with a skull that houses and protects the brain  and sense organs and ribs that protect the heart and  lungs and, in mammals, make breathing possible.  Each of the four limbs is made to the same basic  pattern. It is joined to the spine by means of a flat,  broad bone called a girdle and consists of one long  upper bone, two long lower bones, several smaller  bones in the wrist or ankle and five digits (see  diagrams 6.1 18,19 and 20).   The vertebral column consists of a series of bones called vertebrae linked together to form a flexible column with  the skull at one end and the tail at the other. Each vertebra consists of a ring of bone with spines (spinous process)  protruding dorsally from it. The spinal cord passes through the hole in the middle and muscles attach to the spines  making movement of the body possible (see diagram 6.2).   The shape and size of the vertebrae of mammals vary from the neck to the tail. In the neck there are cervical  vertebrae with the two top ones, the atlas and axis, being specialised to support the head and allow it to nod "Yes"  and shake "No". Thoracic vertebrae in the chest region have special surfaces against which the ribs move during  breathing. Grazing animals like cows and giraffes that have to support weighty heads on long necks have extra large  spines on their cervical and thoracic vertebrae for muscles to attach to. Lumbar vertebrae in the loin region are  usually large strong vertebrae with prominent spines for the attachment of the large muscles of the lower back. The  sacral vertebrae are usually fused into one solid bone called the sacrum that sits within the pelvic girdle. Finally  there are a variable number of small bones in the tail called the coccygeal vertebrae (see diagram 6.3).   The skull of mammals consists of 30 separate bones that grow together during development to form a solid case  protecting the brain and sense organs. The "box "enclosing and protecting the brain is called the cranium (see  diagram 6.4). The bony wall of the cranium encloses the middle and inner ears, protects the organs of smell in the  nasal cavity and the eyes in sockets known as orbits. The teeth are inserted into the upper and lower jaws (see  Chapter 5 for more on teeth) The lower jaw is known as the mandible. It forms a joint with the skull moved by  strong muscles that allow an animal to chew. At the front of the skull is the nasal cavity, separated from the mouth  by a plate of bone called the palate. Behind the nasal cavity and connecting with it are the sinuses. These are air  spaces in the bones of the skull which help keep the skull as light as possible. At the base of the cranium is the  foramen magnum, translated as "big hole", through which the spinal cord passes. On either side of this are two  small, smooth rounded knobs or condyles that articulate (move against) the first or Atlas vertebra.   Paired ribs are attached to each thoracic vertebra against which they move in breathing. Each rib is attached ventrally  either to the sternum or to the rib in front by cartilage to form the rib cage that protects the heart and lungs. In dogs  one pair of ribs is not attached ventrally at all. They are called floating ribs (see diagram 6.5). Birds have a large  expanded sternum called the keel to which the flight muscles (the 'breast" meat of a roast chicken) are attached.   The forelimb consists of: Humerus, radius and ulna, carpals, metacarpals, digits or phalanges (see diagram 6.6).  The top of the humerus moves against (articulates with) the scapula at the shoulder joint. By changing the number,  size and shape of the various bones, fore limbs have evolved to fit different ways of life. They have become wings  for flying in birds and bats, flippers for swimming in whales, seals and porpoises, fast and efficient limbs for running  in horses and arms and hands for holding and manipulating in primates (see diagram 6.8).   The hind limbs have a similar basic pattern to the forelimb. They consist of: femur, tibia and fibula, tarsals,  metatarsals, digits or phalanges (see diagram 6.7). The top of the femur moves against (articulates with) the pelvis  at the hip joint.   This diagram is wrong. The Long pastern or proximal phalanx is PI, not P3. The distal phalanx or coffin bone  (called hoof there) is the 3rd phalanges. The patella or kneecap is embedded in a large tendon in front of the knee. It  seems to smooth the movements of the knee. The legs of the horse are highly adapted to give it great galloping speed   over long distances. The bones of the leg, wrist and foot are greatly elongated and the hooves are actually the tips of  the third fingers and toes, the other digits having been lost or reduced (see diagram 6.9).   The girdles pass on the "push" produced by the limbs to the body. The shoulder girdle or scapula is a triangle of  bone surrounded by the muscles of the back but not connected directly to the spine (see diagram 6.1). This  arrangement helps it to cushion the body when landing after a leap and gives the forelimbs the flexibility to  manipulate food or strike at prey. Animals that use their forelimbs for grasping, burrowing or climbing have a  well-developed clavicle or collar bone. This connects the shoulder girdle to the sternum. Animals like sheep, horses  and cows that use their forelimbs only for supporting the body and locomotion have no clavicle. The pelvic girdle or  hipbone attaches the sacrum and the hind legs. It transmits the force of the leg-thrust in walking or jumping directly  to the spine (see diagram 6.10).   People who study skeletons place the different bones of the skeleton into groups according to their shape or the way  in which they develop. Thus we have long bones like the femur, radius and finger bones, short bones like the ones  of the wrist and ankle, irregular bones like the vertebrae and flat bones like the shoulder blade and bones of the  skull. Finally there are bones that develop in tissue separated from the main skeleton. These include sesamoid bones  which include bones like the patella or kneecap that develop in tendons and visceral bones that develop in the soft  tissue of the penis of the dog and the cow's heart.   Although the skeleton of birds is made up of the same bones as that of mammals, many are highly adapted for flight.  The most noticeable difference is that the bones of the forelimbs are elongated to act as wings. The large flight  muscles make up as much as l/5th of the body weight and are attached to an extension of the sternum called the  keel. The vertebrae of the lower back are fused to provide the rigidity needed to produce flying movements. There  are also many adaptations to reduce the weight of the skeleton. For instance birds have a beak rather than teeth and   A long bone consists of a central portion or shaft and two ends called epiphyses (see diagram 6.12). Long bones  move against or articulate with other bones at joints and their ends have flattened surfaces and rounded  protuberances (condyles) to make this possible. If you carefully examine a long bone you may also see raised or  rough surfaces. This is where the muscles that move the bones are attached. You will also see holes (a hole is called  a foramen) in the bone. Blood vessels and nerves pass into the bone through these. You may also be able to see a  fine line at each end of the bone. This is called the growth plate or epiphyseal line and marks the place where  increase in length of the bone occurred (see diagram 6. 16).   If you cut a long bone lengthways you will see it consists of a hollow cylinder (see diagram 6.13). The outer shell is  covered by a tough fibrous sheath to which the tendons are attached. Under this is a layer of hard, dense compact  bone (see below). This gives the bone its strength. The central cavity contains fatty yellow marrow, an important  energy store for the body, and the ends are made from honeycomb-like bony material called spongy bone (see box   below). Spongy bone contains red marrow where red blood cells are made.   Compact bone is not the lifeless material it may appear at first glance. It is a living dynamic tissue with blood  vessels, nerves and living cells that continually rebuild and reshape the bone structure as a result of the stresses,  bends and breaks it experiences. Compact bone is composed of microscopic hollow cylinders that run parallel to  each other along the length of the bone. Each of these cylinders is called a Haversian system. Blood vessels and  nerves run along the central canal of each Haversian system. Each system consists of concentric rings of bone  material (the matrix) with minute spaces in it that hold the bone cells. The hard matrix contains crystals of calcium  phosphate, calcium carbonate and magnesium salts with collagen fibres that make the bone stronger and somewhat  flexible. Tiny canals connect the cells with each other and their blood supply (see diagram 6.14).   Spongy bone gives bones lightness with strength. It consists of an irregular lattice that looks just like an old  fashioned loofah sponge (see diagram 6.15). It is found on the ends of long bones and makes up most of the bone  tissue of the limb girdles, ribs, sternum, vertebrae and skull. The spaces contain red marrow, which is where red  blood cells are made and stored.   The skeleton starts off in the foetus as either cartilage or fibrous connective tissue. Before birth and, sometimes for  years after it, the cartilage is gradually replaced by bone. The long bones increase in length at the ends at an area  known as the epiphyseal plate where new cartilage is laid down and then gradually converted to bone. When an  animal is mature, bone growth ceases and the epiphyseal plate converts into a fine epiphyseal line (see diagram  6.16).   A fracture or break dramatically demonstrates the dynamic nature of bone. Soon after the break occurs blood pours  into the site and cartilage is deposited. This starts to connect the broken ends together. Later spongy bone replaces  the cartilage, which is itself replaced by compact bone. Partial healing to the point where some weight can be put on  the bone can take place in 6 weeks but complete healing may take 3-4 months.   Joints are the structures in the skeleton where 2 or more bones meet. There are several different types of joints. Some  are immovable once the animal has reached maturity. Examples of these are those between the bones of the skull  and the midline joint of the pelvic girdle. Some are slightly moveable like the joints between the vertebrae but most  joints allow free movement and have a typical structure with a fluid filled cavity separating the articulating surfaces  (surfaces that move against each other) of the two bones. This kind of joint is called a synovial joint (see diagram  6.17). The joint is held together by bundles of white fibrous tissue called ligaments and a fibrous capsule encloses  the joint. The inner layers of this capsule secrete the synovial fluid that acts as a lubricant. The articulating surfaces  of the bones are covered with cartilage that also reduces friction and some joints, e.g. the knee, have a pad of  cartilage between the surfaces that articulate with each other.   The shape of the articulating bones in a joint and the arrangement of ligaments determine the kind of movement  made by the joint. Some joints only allow a to and from gliding movement e.g. between the ankle and wrist bones;  the joints at the elbow, knee and fingers are hinge joints and allow movement in two dimensions and the axis   vertebra pivots on the atlas vertebra. Ball and socket joints, like those at the shoulder and hip, allow the greatest  range of movement.   Some joints in animals are given common names that tend to be confusing. For example:   1 . The joint between the femur and the tibia on the hind leg is our knee but the stifle in animals.   2. Our ankle joint (between the tarsals and metatarsals) is the hock in animals   3. Our knuckle joint (between the metacarpals or metatarsals and the phalanges) is the fetlock in the horse.   4. The "knee" on the horse is equivalent to our wrist (ie on the front limb between the radius and metacarpals)  see diagrams 6.6, 6.7, 6.8, 6.17 and 6.18.   Different animals place different parts of the foot or forelimb on the ground when walking or running.   Humans and bears put the whole surface of the foot on the ground when they walk. This is known as plantigrade  locomotion. Dogs and cats walk on their toes (digitigrade locomotion) while horses and pigs walk on their  "toenails" or hoofs. This is called unguligrade locomotion (see diagram 6.20).   1 . Plantigrade locomotion (on the "palms of the hand) as in humans and bears   2. Digitigrade locomotion (on the "fingers") as in cats and dogs   • The skeleton maintains the shape of the body, protects internal organs and makes locomotion possible.   • The vertebrae support the body and protect the spinal cord. They consist of: cervical vertebrae in the neck,  thoracic vertebrae in the chest region which articulate with the ribs, lumbar vertebrae in the loin region, sacral  vertebrae fused to the pelvis to form the sacrum and tail or coccygeal vertebrae.   • The skull protects the brain and sense organs. The cranium forms a solid box enclosing the brain. The mandible  forms the jaw.   • The forelimb consists of the humerus, radius, ulna, carpals, metacarpals and phalanges. It moves against or  articulates with the scapula at the shoulder joint.   • The hindlimb consists of the femur, patella, tibia, fibula, tarsals, metatarsals and digits. It moves against or  articulates with the pelvis at the hip joint.   • Compact bone in the shaft of long bones gives them their strength. Spongy bone at the ends reduces weight.  Bone growth occurs at the growth plate.   Use the Skeleton Worksheet to learn the main parts of the skeleton.   5. Attach the following labels to the diagram of the long bone shown below.   • http://www.infovisual.info/02/056_en.html Bird skeleton  A good diagram of the bird skeleton.   A great introduction to the mammalian skeleton. A little above the level required but it has so much interesting  information it's worth reading it.   • http://www.klbschool.org.uk/interactive/science/skeleton.htm The human skeleton  Test yourself on the names of the bones of the (human) skeleton.   • http://www.shockfamily.net/skeleton/JOINTS.HTML The joints  Quite a good article on the different kinds of joints with diagrams.   Wikipedia is disappointing where the skeleton is concerned. Most articles stick entirely to the human skeleton or  have far too much detail. However this one on compact and spongy bone and the growth of bone is quite good  although still much above the level required.   b) the thoracic vertebrae: move against adjacent cranial and caudal vertebrae & the ribs   From the scapula are the humerus, radius, ulna, carpals, metacarpals and digits.   The patella is the knee cap. It is found in the hind limb at the knee joint.   a) The stifle joint: between the femur and the tibia on the hind limb.   b) The elbow joint: between the humerus, radius and ulna on the forelimb.   c) The hock joint: between the tarsals and metatarsals on the hind limb.   5. Attach the following labels to the diagram of the long bone shown below.   • the structure of smooth, cardiac and skeletal  muscle and where they are found   Muscles make up the bulk of an animal's body and  account for about half its weight. The meat on the  chop or roast is muscle and is composed mainly of  protein. The cells that make up muscle tissue are  elongated and able to contract to a half or even a  third of their length when at rest. There are three  different kinds of muscle; smooth, cardiac and  skeletal muscle.   moving food down the digestive system, keeping the eyes in focus and adjusting the diameter of blood vessels. The   individual cells are spindle-shaped, being fatter in the middle and tapering off towards the ends with a nucleus in the   centre of the cell. They are usually found in sheets and are stimulated by the non-conscious or autonomic nervous   Cardiac muscle is only found in the wall of the heart. It is composed of branching fibres that form a  three-dimensional network. When examined under the microscope, a central nucleus and faint stripes or striations  can be seen in the cells. Cardiac muscle cells contract spontaneously and rhythmically without outside stimulation  but the pacemaker coordinates the heart beat. Nerves and hormones modify this rhythm (see Chapter 3).   Skeletal muscle is the muscle that is attached to and moves the skeleton, and is under voluntary control. It is  composed of elongated cells or fibres lying parallel to each other. Each cell is unusual in that it has several nuclei  and when examined under the microscope appears striped or striated. This appearance gives the muscle its names of  striped or striated muscle. Each cell of striated muscle contains hundreds, or even thousands, of microscopic fibres  each one with its own striped appearance. The stripes are formed by two different sorts of protein that slide over each  other making the cell contract (see diagram 7.1).   Muscle contraction requires energy and muscle cells have numerous mitochondria. However, only about 15% of the  energy released by the mitochondria is used to fuel muscle contraction. The rest is released as heat. This is why  exercise increases body temperature and makes animals sweat or pant to rid themselves of this heat.   What we refer to as a muscle is made up of groups of muscle fibres surrounded by connective tissue. The connective  tissue sheaths join together at the ends of the muscle to form tough white bands of fibre called tendons. These attach  the muscles to the bones. Tendons are similar in structure to the ligaments that attach bones together across a joint  (see diagrams 7.2a and b).   A single muscle is fat in the middle and tapers towards the ends. The middle part, which gets fatter when the muscle  contracts, is called the belly of the muscle. If you contract your biceps muscle in your upper arm you may feel it  getting fatter in the middle. You may also notice that the biceps is attached at its top end to bones in your shoulder  while at the bottom it is attached to bones in your lower arm. Notice that the bones at only one end move when you  contract the biceps. This end of the muscle is called the insertion. The other end of the muscle, the origin, is  attached to the bone that moves the least (see diagram 35).   Skeletal muscles usually work in pairs. When one contracts the other relaxes and vice versa. Pairs of muscles that  work like this are called antagonistic muscles. For example the muscles in the upper forearm are the biceps and  triceps (see diagram 7.3). Together they bend the elbow. When the biceps contracts (and the triceps relaxes) the  lower forearm is raised and the angle of the joint is reduced. This kind of movement is called flexion. When the  triceps is contracted (and the biceps relaxes), the angle of the elbow increases. The term for this movement is  extension.   When you or animals contract skeletal muscle it is a voluntary action. For example, you make a conscious decision  to walk across the room, raise the spoon to your mouth or smile. There is however, another way in which contraction  of muscles attached to the skeleton happens that is not under voluntary control. This is during a reflex action, such  as jerking your hand away from the hot stove you have touched by accident. This is called a reflex arc and will be  described in detail in chapter 14-15.   • There are three different kinds of muscle tissue: smooth muscle in the walls of the gut and blood   vessels; cardiac muscle in the heart and skeletal muscle attached to the skeleton.   • Skeletal muscles work in pairs known as antagonistic pairs. As one contracts the other in the   • Flexion is the movement that reduces the angle of a joint. Extension increases the angle   6. When you extend your ankle joint what happens?  Test Yourself Answers   • http://health.howstuffworks.com/muscle.htm How muscles work  Description of the three types of muscles and how skeletal muscles work.   d) Smiling is a voluntary action (usually!) so skeletal muscle is involved.   e) You cannot control "Goosebumps" so the muscles involved are smooth muscles.   f) Although breathing seems to be involuntary most of the time it is possible to control it voluntarily so the  diaphragm (the main breathing muscle) is a skeletal muscle.   3. What is the insertion of a muscle? The insertion of a muscle is the end attached to the bone that moves most when  the muscle contracts.   The triceps is the muscle that forms the other half of the antagonistic pair with the biceps.   This chapter on the Cardiovascular system is divided  into 3 sections. These are:   the appearance and function of white blood cells particularly granulocytes, lymphocytes and   • how oxygen and carbon dioxide are transported in the blood   • the names of some anticoagulants and their function in the body and in the vet clinic   Blood is a unique fluid containing cells that is pumped by the heart around the body of animals in a system of pipes  known as the circulatory system. It carries oxygen and nutrients to the cells of the body and removes waste  products like carbon dioxide from them. Blood is also important for keeping conditions in the body constant, in other  words for maintaining homeostasis. It helps keep the acidity or pH stable and helps maintain a constant temperature  in the body. Blood also has an important role in defending the body against disease.   A simple way to find out what is in blood is to remove a small amount from an animal and place it in a tube with a  substance that prevents it from clotting (an anticoagulant). If you leave the tube to stand for a few hours you will  find that it settles out into two layers. The top layer consists of a light yellow fluid, the plasma, and the bottom layer  consists of red blood cells (RBCs). If you look very carefully you can also see a thin beige-coloured layer in  between these two layers. This consists of the white blood cells (WBCs) (see diagram 8.1).   The above procedure is usually done more rapidly by placing the blood sample in a centrifuge for a few minutes.  This machine acts like a super spin drier rotating about 10,000 times a minute and packing the heavier particles (red  blood cells) at the bottom of the tube. The sample that results is called the packed cell volume (P.C.V.) or  haematocrit. It is a very useful measurement of the concentration of red blood cells in the blood. For most animals  the packed cell volume is in the range 30-45%. If it is lower than this it means that the concentration of red blood  cells is low and the animal is anaemic. If the reading is above this range it may mean the animal is dehydrated.  Animals that live at high altitudes also have high P.C.V.s to compensate for the low oxygen concentration there.   Plasma consists of water (91%) in which many substances are dissolved. These dissolved substances include:   Salts in the plasma are in the form of ions or electrolytes which include sodium, potassium, calcium, chloride,  phosphate and bicarbonate. Plasma transports these ions to where they are needed e.g. calcium required by the bones,  they also help keep the osmotic pressure and acid-base balance (pH) of the blood within the required levels.   The proteins in the blood plasma are large molecules with important functions. Some contribute to the osmotic  pressure (see chapter 3) and the viscosity (thickness) of the blood, and so help keep the blood volume and pressure  stable. Others act as antibodies that attack bacteria and viruses, and yet others are important in blood clotting.  Nutrients that are absorbed from the gut and transported to the cells in the plasma include amino acids, glucose, fatty  acids and vitamins. Waste products include urea from the breakdown of proteins.   Red blood cells are also known as RBCs or erythrocytes. They are what make blood red. When you look at a blood  smear through a microscope, as you will in one of the practical classes, you will see that RBCs are by far the most  common cells in the blood. (In fact there are about 5 million per millilitre). If you focus on an individual RBC you  will see that they are shaped like discs or doughnuts with a thin central portion surrounded by a fatter margin. This  shape has all sorts of advantages, one being that enables the cells to fold up and pass along the narrowest blood  capillaries (See diagram 8.2).   The mature RBCs of mammals have neither nucleus nor other organelles and can be thought of as sacks of  haemoglobin. Haemoglobin is a red coloured protein containing iron, which joins with oxygen so the blood can  transport it to body cells. RBCs are made continuously in the bone marrow and live about 120 days. They are then  destroyed in the liver and spleen and the molecules they are made from recycled to make new RBCs. Anaemia  results if the rate at which RBCs are destroyed exceeds the rate at which RBC'c are produced.   Note that if you happen to look at bird's, reptiles, frogs or fishes blood down the microscope you will see that these  vertebrates all have RBCs with a central nucleus.   White blood cells or leucocytes are far less numerous than red blood cells. In fact there is only about one white cell  for every 1000 red blood cells. Rather than being white, they are actually colourless as they contain no haemoglobin  although unlike RBCs they do have a nucleus. If you make a blood smear and look at it under the microscope it is  difficult to see the white blood cells at all. To make them visible you need to stain them with special dyes or stains.  There are a variety of stains that can be used, but most dye the nucleus a dark purple or pink colour. The stains may  also show up the granules present in the cytoplasm of some white blood cells. White blood cells are divided into two  major groups depending on the shape of the nucleus and whether or not there are granules in the cytoplasm.   1. Granulocytes or polymorphonuclear leucocytes ("polymorphs" or "polys") have granules in the cytoplasm  and a purple lobed nucleus (see diagram 8.3). The most common (neutrophils) can squeeze out of capillaries  and are involved in engulfing and destroying foreign invaders like bacteria (see diagram 8.4). Some  (eosinophils) combat allergies and increase in numbers during parasitic worm infections. Others (basophils)  produce heparin that prevents the blood from clotting.   2. Agranulocytes or monomorphonuclear leucocytes have a large unlobed nucleus and no granules in the  cytoplasm. There are two types of agranulocytes. The most numerous are lymphocytes that are concerned  with immune responses. The second type is the monocyte that is the largest blood cell and is involved in  engulfing bacteria etc. by phagocytosis (see diagram 8.5).   As well as red and white blood cells, the blood also contains small irregular shaped fragments of cells known as  platelets. They are involved in the clotting of the blood (see later).   The purpose of the haemoglobin in red blood cells is to carry oxygen from the lungs to the tissues. In fact it allows  the blood to carry about 25 times more oxygen than it would be able to without any haemoglobin.   When oxygen concentrations are high, as in the blood capillaries in the lungs, haemoglobin combines with oxygen to  form a compound called oxyhaemoglobin. This compound is bright red and makes the oxygenated blood that spurts  from a damaged artery its characteristic bright red colour. When the blood reaches the tissues where the oxygen  concentrations are low, the oxygen separates from the haemoglobin and diffuses into the tissues. The haemoglobin in  most veins has given up its oxygen and the blood is called deoxygenated blood. It is a purple-red colour.   Carbon monoxide is a colourless, odourless gas found in car exhaust fumes and tobacco smoke. It combines with  haemoglobin just like oxygen but does not let go. This means the haemoglobin molecules are not available to carry  oxygen to the tissues and the animal or human suffocates. Carbon monoxide poisoning is often fatal but can be  treated by giving the patient pure oxygen that slowly replaces the carbon monoxide.   Carbon dioxide is a waste gas produced by cells. It diffuses into the blood capillaries where it is carried to the lungs  in the blood. Most is carried in the plasma as bicarbonate ions but a small amount is dissolved directly in the  plasma and some combines with haemoglobin.   The blood carries water to the cells and organs as well as soluble food substances (sugars, amino acids, fatty acids  and vitamins) and hormones dissolved in the plasma. These are delivered to the cells via the tissue fluid (see later in  this chapter) that surrounds them. Blood also picks up the waste products like carbon dioxide and urea from the cells  and is important in distributing the heat produced in the liver and muscles all over the body.   The mechanism that causes the blood to clot is easily seen when you or your animals are injured. However, minor  injuries occur all the time in areas that experience wear and tear like the intestine, the lungs and the skin. Without the  clotting mechanism, animals would quickly bleed to death from minor injury and internal haemorrhage. This is  what happens in animals and people with clotting disorders like haemophilia, as well as animals that are poisoned  with rat poisons like warfarin.   Platelets are important in blood clotting. When blood vessels are damaged, substances released cause the blood  platelets to disintegrate. This stimulates a complex chain of reactions, which causes the protein fibrinogen to be  converted to fibrin. Fibrin forms a dense fibrous network over the wound preventing the escape of further blood.  Calcium and vitamin K are essential for the clotting process and any deficiency of these may also lead to clotting  problems.   When blood clots it separates into the clot that contains most of the cells and platelets leaving behind a straw  coloured fluid. This fluid is called serum. It looks just like plasma and is similar in composition except for one big  difference. It doesn't contain fibrinogen, the protein that forms the clot.   Anticoagulants are substances that interfere with the clotting process. When blood is collected for transfusion or  testing it is often important to prevent it clotting and there are a number of different anticoagulants you can use for  this. Tubes containing the different anticoagulants are coded with different colours for easy recognition.   1 . Heparin (colour code - green) is a natural anticoagulant produced by the white blood cells but it is also used  routinely in the laboratory with samples to be tested for heavy metals like lead.   4. Citrate (colour code — light blue) is used for the storage of large quantities of blood, such as used in  transfusions.   Haemolysis is the breakdown of the plasma membrane of red blood cells to release the haemoglobin. We have  already met this process when discussing osmosis, for haemolysis often occurs when red blood cells are placed in a  hypotonic solution and water flows in through the semi permeable plasma membrane to swell and eventually burst  the cell. It is therefore important when collecting blood from an animal to make sure there is no water in the syringe  or tube. Too much movement due to shaking the tube or sucking up the blood too vigorously can also break down  the plasma membrane and cause haemolysis.   If you have given blood recently you may know your blood group. It may be blood group O, A or B or even AB, the  rarest group. Blood groups are the result of different molecules called antigens on the outside of red blood cells.  These cause antibodies to be formed that attack viruses and bacteria. Knowledge of a person's blood group is  important when giving transfusions because if blood of another incompatible blood group is given to a patient the  red blood cells stick together and block the blood vessels and may lead to death.   Blood groups also exist in many animals. There are three blood groups in cats and great care has to be taken that the  groups are compatible when transfusing exotic breeds. The situation is slightly different in dogs. They have a  number of blood groups but there is usually no problem with the first blood transfusion a dog receives. However, this  first transfusion sensitises the immune system so that a problem may arise with the second and subsequent  transfusions.   Haemolysis can occur in the living animal when it is exposed to various poisons and toxins. This may happen when,  for example, it eats a poisonous plant, is bitten by a snake or infected with bacteria that destroy red blood cells  (haemolytic bacteria).   Blood accounts for between 6-10% of the body weight of animals, varying with the species and the stage of life.  Animals can not tolerate losses of greater than 3% of the total volume when the condition known as shock occurs.   • The main functions of blood are transport of oxygen, food, waste products etc., the maintenance of homeostasis  and defending the body from disease.   • Blood consists of fluid, plasma, in which red and white blood cells are suspended. The blood cells typically make  up 30-45% of the blood volume.   • Plasma consists of water containing dissolved substances like proteins, nutrients and carbon dioxide.   • White Blood Cells defend the body from invasion. There are 2 kinds:   • Granular white cells include neutrophils, basophils and eosinophils. Neutrophils which destroy bacteria are the  most numerous. Eosinophils are involved with allergies and parasitic infections.   • Non-granular white cells include lymphocytes that produce antibodies to attach bacteria and viruses and  monocytes that engulf and destroy bacteria and viruses.   The exercises in the Blood Worksheet will help you learn how to identify the different types of blood cell and  what their functions are.   2. There are two main types of cells in blood. They are:   5. How would you tell a white cell from a red cell when looking at them through a microscope? (Give at least 2  differences)   6. How does the blood help fight invasion by bacteria and viruses?   7. What would happen to blood if there were no platelets?  Test Yourself Answers   Shows constituents of blood including RBCs, white cells, platelets and plasma. Even shows how to make a blood  smear and identify the white cells on it as well as make and read a haematocrit. Some parts are a little too advanced.   • Wikipedia has good information of red and white blood cells.   1 . The liquid part of blood is known as plasma   2. There are two main types of cells in blood. They are:   3. The most numerous cells in blood are red blood cells   4. The main function of the red blood cells is to transport oxygen.   5. How would you tell a white cell from a red cell when looking at them through a microscope? (Give at least 2  differences)   You would tell a white cell from a red cell when looking at them through a microscope by the red (pink) colour  and no nucleus of red blood cells compared to the white cells which have a colourless cytoplasm and a purple  stained nucleus.   6. How does the blood help fight invasion by bacteria and viruses?   White cells in the blood help fight invasion by bacteria and viruses by engulfing them (neutrophils and  monocytes) or producing chemicals (antibodies) that kill them (lymphocytes).   7. What would happen to blood if there were no platelets?   If there were no platelets in the blood it would not clot to stop bleeding.   • the structure and function of the heart valves and their role in producing the heart sounds   • the stages of the heart beat and the route the blood takes through the heart   The heart is the pump that pushes the blood  around the body in the blood vessels of the  circulatory system. In fishes the blood  only passes through the heart once on its  way to the gills and then round the rest of  the body. However, in mammals and birds  that have lungs, the blood passes through the  heart twice: once on its way to the lungs  where it picks up oxygen and then through  the heart again to be pumped all over the  body. The heart is therefore two separate  pumps, side by side (see diagram 8.6).   The heart is situated in the thorax between  the lungs and is protected by the rib cage. In  some animals it is displaced slightly to the  left-hand side. A tough membrane called the  pericardium covers it. There is a narrow  space between the pericardium and the heart that is filled with a liquid that acts as a lubricant.   The heart of mammals is a hollow bag made of cardiac muscle (see chapter 4). The cavity inside the heart is divided  into 4 chambers. The chambers on the right side are completely separate from the chambers on the left side. The two  upper chambers are thin walled, and are called the atria (or auricles). The two lower chambers are thick walled and  are called the ventricles (see diagrams 8.7 and 8.8).   Diagram 8.7: Simplified diagram of the internal  structure of the mammalian heart    Diagram 8.8: The internal structure and blood  flow through the heart   Blood flows through the heart in a one way system. The right atrium receives deoxygenated blood from the body via  the largest vein in the body called the vena cava. The contraction of the atrium pumps the blood into the right  ventricle and then into the lungs via the pulmonary artery. The blood is oxygenated in the lungs and then returns to  the heart and enters the left atrium via the pulmonary vein. The contraction of the left atrium pumps the blood into  the left ventricle, which then pumps it to the body via the aorta (see diagrams 8.7 and 8.8). The wall of the left  ventricle is usually much thicker than that of the right ventricle because it has to pump the blood to the end of the  digits and tip of the tail while the right ventricle only has to pump the blood to the nearby lungs.   Valves are flaps of tissue that stop blood flowing backwards and so control the direction of blood flow in the heart.  There are two kinds of valves in the heart. The first kind is the massive valves between the atria and the ventricles,  the atrio-ventricular valves, (AV valves) that prevent blood in the ventricles from flowing back into the atria. The  flaps of these valves are attached to the walls of the ventricles by tendons. These make them look somewhat like  parachutes (see diagram 8.9).   The second kind of valve is pocket shaped flaps of tissue called the semilunar (half moon) valves (see diagram 8).  They are called the pulmonary and aortic valves and found at the back of the pulmonary artery and aorta  respectively.   The heartbeat consists of alternating contractions and relaxations of the heart. It you listen to the heart with a  stethoscope you hear the sounds often described as "lubb-dupp".   1. Each atrium relaxes so that blood can enter. Blood flows from the body via the vena cava into the :right  atrium. At the same time, blood flows from the lungs via the pulmonary vein into the left :atrium (see diagram  8.10a).   2. The atrio-ventricular valves open and both ventricles relax. The atria contract and blood flows from the right  atrium into the right ventricle and from the left atrium into the left ventricle (see diagram 8.10b).   3. The ventricles contract and the atrio-ventricular valves snap shut to stop blood flowing back into the atria. This  is the first sound ("lubb") of the heartbeat that can be heard with a stethoscope (see diagram 8.10c).   4. The semi-lunar valves open and blood is pumped out of the right ventricle to the lungs. At the same time,  blood is pumped out of the left ventricle into the aorta and so to the rest of the body. When the ventricles stop  contracting the semi-lunar valves snap shut to stop blood flowing backwards.   This is the second sound ("dupp") of the heartbeat. Blood flows into the atria again as they relax and the cycle is  repeated.   When a valve is damaged and fails to close completely some blood may flow backwards after each heartbeat. A  trained veterinarian hears this with a stethoscope as a "heart murmur".   The period of the heart beat when the ventricles are contracting and sending a wave of blood down the pulmonary  artery and aorta is called systole. The period when the ventricles are relaxing is called diastole.   The walls of the heart consist of cardiac muscle, a special kind of muscle only found in the heart. The cells of cardiac  muscle form a branching network of separating and rejoining fibres which allows nerve impulses to travel through  the tissue (see Chapter 7). Heart muscle needs lots of energy to function so it is well supplied with mitochondria and  requires a good supply of oxygen. This is provided by the coronary arteries (see below).   Diagram 8.10 a) First stage of heartbeat b) Second stage of heartbeat c) Third stage of heartbeat   The cardiac muscle of the walls of the heart contracts of its own accord. This can be demonstrated by the rather  macabre experiment in which a small portion of heart muscle is removed and placed in a solution that is similar to  blood. The tissue will continue to contract and relax for a time. In the normal functioning heart the pacemaker acts  rather like the conductor of an orchestra and superimposes a unified beat upon the heart as a whole. The pacemaker  is situated in the wall of the right atrium. The rate at which the heart beats is modified by a part of the brain called  the medulla oblongata (see Chapter 14) and by the hormone adrenalin (see Chapter 16) which speeds up the  heartbeat.   Although oxygenated blood passes through some of the chambers of the heart it can not supply the muscle of the  heart walls with the oxygen and nutrients it needs. Special arteries called the coronary arteries do this. These two  arteries arise from the aorta and branch through the heart to deliver oxygen and nutrients to the cardiac muscles and  collect carbon dioxide and wastes. Coronary veins return the blood to right hand side of the heart. Some of these  vessels can be seen on the outside surface of the heart (see diagram 8.11). Sometimes fatty deposits on the inside  wall of the coronary artery block the blood flow to the heart muscle. If the obstruction is severe enough to damage  the heart muscle due to inadequate blood supply a "heart attack" can result.   • The heart is situated in the thorax between the lungs   • The heart is a hollow bag made of cardiac muscle. It is divided into four chambers (right and left atria and right  and left ventricles).   • Valves stop blood flowing backwards. The right and left atrio-ventricular valves prevent blood in the ventricles  from flowing back into the atria. The semilunar valves at the entrance of the pulmonary artery and aorta prevent  blood flowing back into the ventricles. The closing of the valves produces the heart sounds heard with a  stethoscope.   • There are 4 stages to the heart beat. 1. blood flows into the right and left atria. 2. The atria contract and blood  flows into the ventricles. 3. The ventricles contract and the closing of the atrio-ventricular valves produces the  first heart sound. 4. Blood flows to the lungs and body and when the ventricles stop contracting the closing of the   • The coronary arteries supply the heart muscle with oxygenated blood.   Use the Heart Worksheet to help you learn the different parts of the heart, the role of the heart valves and how the  heart beat pushes the blood through the heart.   1. Via what vessel does blood enter the heart from the body?   2. Blood passing through the right atrioventricular valve passes into which chamber of the heart?   5. The walls of the heart are made of what tissue?   An animation of blood flow through the heart with step-by-step annotated breakdown. Also an animation putting  heartbeat, valve operation and blood flow all together.   Great animation showing parts of heart plus constituents of blood including RBCs, white cells, platelets and plasma.  Even shows how to make a blood smear and identify the white cells on it as well as make and read a haematocrit.  Some parts are a little too advanced.   Drag and drop animation of the human heart. Great for revision but note the terms bicuspid and tricuspid valves are  used. These are equivalent to the left and right atrio-ventricular valves in animals.   A good animation of the heartbeat showing the valves opening and closing with appropriate heart sounds. Plus a  good diagram showing the difference between the atrio-ventricular and semilunar valves.   A great animation where you use the mouse to point to parts of the heart and a voice over tells you what part it is.  Just note this is of the human heart so the terms superior and inferior vena cava are used instead of caudal and cranial  as for an animal.   Again Wikipedia is a wonderful resource, although remember, most of the material is on the human system.   1. Via what vessel does blood enter the heart from the body?   Blood enter the heart from the body via the cranial and caudal vena cavae.   2. Blood passing through the right atrioventricular valve passes into which chamber of the heart?   The pulmonary valve prevents backflow of blood from the pulmonary artery into the right ventricle.   5. The walls of the heart are made of what tissue?   • that the circulatory system is double consisting of pulmonary and systemic circuits with the blood :passing  through the heart twice;   • the differences in the structure and function of arteries, capillaries and veins;   • how the pulse is produced and where it can be felt;   • what tissue fluid and lymph are and how they are formed;   The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It  supplies the tissues with their requirements and removes waste products. In mammals and birds the blood circulates  through two separate systems - the first from the heart to the lungs and back to the heart again (the pulmonary  circulation) and the second from the heart to the head and body and back again (the systemic circulation) (see  diagram 8.12).   The tubes through which the blood flows are the arteries, capillaries and veins. The heart pumps blood into arteries  that carry it away from the heart. The arteries divide into very thin vessels called capillaries that form a network   between the cells of the body. The capillaries then join up again to make veins that return the blood to the heart.   Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the surges of  high pressure blood caused by the heartbeat (the pulse, see later). The arteries divide into smaller vessels called  arterioles. The hole down the centre of the artery is called the lumen. There are three layers of tissue in the walls of  an artery. It is lined with squamous epithelial cells. The middle layer is the thickest layer. It made of elastic fibres  and smooth muscle to make it stretchy. The outer fibrous layer protects the artery (see diagram 8.13). The pulse is  only felt in arteries.   The pulse is the spurt of high pressure blood that passes along the aorta and arteries when the left ventricle contracts.  As the pulse of blood passes along an artery the elastic walls stretch. When the pulse has passed the walls contract  and this helps push the blood along. The pulse is easily felt at certain places where an artery passes near the surface  of the body. It is strongest near the heart and becomes weaker as it travels away from the heart. The pulse disappears  altogether in the capillaries.   Arterioles divide repeatedly to form a network penetrating between the cells of all tissues of the body. These small  vessels are called capillaries. The walls are only one cell thick and some capillaries are so narrow that red blood cells  have to fold up to pass through them. Capillaries form networks in tissues called capillary beds. The capillary  networks in capillary beds are so dense that no living cell is far from its supply of oxygen and food (see diagram  8.14).   Note: All arteries carry oxygenated blood except for the pulmonary artery that carries deoxygenated blood to the  lungs.   The thin walls of capillaries allow water, some white blood cells and many dissolved substances to diffuse through  them. These form a clear fluid called tissue fluid (or extracellular fluid or interstitial fluid) that surrounds the cells  of the tissues. The tissue fluid allows oxygen and nutrients to pass from the blood to the cells and carbon dioxide and  other waste products to be removed from the tissues (see diagram 8.15).   Diagram 8.15 - The formation of tissue fluid and lymph from blood   Some tissue fluid finds its way back into the capillaries and some of it flows into the blind-ended lymphatic vessels  that form a network in the tissues. Once the tissue fluid has entered the lymphatics it is called lymph although its  composition remains the same. The lymph vessels have walls that are even thinner than the capillaries. This means  that molecules and particles that are larger than those that can pass into the blood stream e.g. cancer cells and  bacteria can enter the lymphatic system. These are then filtered out as the lymph passes through lymph nodes. (See  chapter 10 for more information on the lymphatic system).   Capillaries unite to form larger vessels called venules that join to form veins. Veins return blood to the heart and  since blood that flows in veins has already passed through the fine capillaries, it flows slowly with no pulse and at  low pressure. For this reason veins have thinner walls than arteries although there have the same three layers in them  as arteries (see diagram 8.16). As there is no pulse in veins, the blood is squeezed along them by the contraction of  the skeletal muscles that lay alongside them.   Veins also have valves in them that prevent blood flowing backwards (see diagram 8.17a and b).   Diagram 8.16 - Cross section of a vein  a) valve open b) valve closed   Note: Most veins carry deoxygenated blood. The pulmonary vein that carries oxygenated blood from the lungs to the  left atrium of the heart is an exception.   The flow of blood along arteries, arterioles and capillaries is not constant but can be controlled depending upon the  requirements of the body. For example more blood is directed to the skeletal muscles, brain or digestive system  when they are active. Regulation of the blood flow to the arterioles of the skin is also important in controlling body  temperature. The size of the vessels is adjusted by the contraction or relaxation of smooth muscle fibres in their  walls.   Oedema is the swelling of the tissues due to the accumulation of tissue fluid. This may occur because the tissue fluid  is prevented from returning to the bloodstream and accumulates in the tissues. This may be caused by physical  inactivity (e.g. long car or plane trips in humans) or because of imbalances in the proteins in the blood. This is what  causes the "pot-belly" of the malnourished child or worm-infested puppy.   Loss of body fluid can be caused not only by drinking insufficient liquid but also through diarrhea and vomiting or  sudden loss of blood due to haemorrhage. The effect is to reduce the volume of the blood which decreases the blood  pressure. This could be dangerous because the supply of adequate blood to the brain depends upon maintaining the  blood pressure at a constant level.   To compensate for the loss of fluid various mechanisms come into play. First of all the blood vessels contract in  order to try and maintain the pressure. Then, since the loss of fluid tends to make the blood more concentrated and  increases its osmotic pressure, fluid is drawn into the blood from the tissues by osmosis.   The spleen is situated near the stomach. It has a rich blood supply and acts as a reservoir of red blood cells. When  there is a sudden loss of blood, as happens when a haemorrhage occurs, the spleen contracts to release large numbers  of red blood cells into the circulation. The spleen also destroys old red cells and makes new lymphocytes but it is not  an essential organ because its removal in adult life seems to cause few problems. In the foetus, the spleen makes both  red and white cells.   Blood is pumped out into the body via the main artery, the aorta. This takes the blood to the head, the limbs and all  the body organs. After passing through a network of fine capillaries, the blood is returned to the heart in the largest  vein, the vena cava (see diagrams 8.8, 8.12, 8.18 and 8.19).   Arteries and veins to and from many organs often run alongside each other and have the same name e.g. the renal  artery and vein serve the kidney, the femoral artery and vein serve the hind limbs and the subclavian artery and  vein serve the forelimbs. However, blood to the head passes along the carotid artery and returns to the cranial vena  cava via the jugular vein.   One variation on this arrangement is found in the blood vessels that serve the digestive tract. A variety of arteries  take blood from the aorta to the intestines but blood from the intestines is carried by the hepatic portal vein to the  liver where the digested food can be processed (see diagram 8.12). This vessel is unlike others in that it transports  blood from one organ to another rather than to or from the heart like arteries or veins.   Diagram 8.18 - The main arteries and veins of the horse   The blood pressure is the pressure of the blood against the walls of the main arteries. The pressure is highest as the  pulse produced by the contraction of the left ventricle passes along the artery. This is known as the systolic  pressure. Pressure is much lower between pulses. This is known as the diastolic pressure. Blood pressure is  measured in millimetres of mercury. A blood pressure that is higher than expected is known as hypertension while a  pressure lower than expected is known as hypotension.   artery to stomach  artery to small intestine  artery to large intestine  artery to ovary or testis   • The circulatory system is double with the blood passing through the heart twice.   • Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the high  pressure of the pulse.   • Capillaries are small, thin walled vessels that form a network between the cells of the tissues.   • Veins return low pressure blood to the heart. They have thinner walls than arteries.   • The pulse is the spurt of high pressure blood that passes along the arteries when the left ventricle contracts. It can  be felt where arteries pass close to the body surface.   • Tissue fluid is the clear fluid that leaks from the capillaries and surrounds the cells of the tissues. Lymph forms  when tissue fluid enters lymphatics.   • Important blood vessels include the vena cava, aorta, pulmonary artery, carotid artery, jugular vein, renal artery  and vein and hepatic portal vessel.   This Circulatory System Worksheet will help you learn the main vessels of the circulatory system, the difference  between arteries and veins, and what happens in a capillary bed.   Blood flows by this route through the blood vessels of the body.   from the heart to the main organs of the body:  from the aorta to the brain:  from the aorta to the kidneys:  from the intestines to the liver:  from the aorta to the heart muscle:  from the aorta to the heart muscle:  Test Yourself Answers   • Wikipedia has good information on the circulatory system, although remember, most of the material is on the  human system.   2. Give 2 other differences between arteries and veins. Arteries have thicker walls than veins. Veins have valves to  prevent backflow of blood.   Systole is the period when the ventricles are contracting and sending a wave of high-pressure blood (the  pulse) along the aorta and arteries of the body.   4. Circle the correct answer below. Blood flows by this route through the blood vessels of the body.   • why animals need energy and how they make it in  cells   • why animals require oxygen and need to get rid of  carbon dioxide   • the structure of alveoli and how oxygen and  carbon dioxide pass across their walls   • how oxygen and carbon dioxide are carried in the  blood   • the route air takes in the respiratory system (i.e.  the nose, pharynx, larynx, trachea, bronchus,   • the movements of the ribs and diaphragm to bring  about inspiration   • how the rate of breathing is controlled and how  this helps regulate the acid-base balance of the  blood   Animals require a supply of energy to survive. This energy is needed to  build large molecules like proteins and glycogen, make the structures  in cells, move chemicals through membranes and around cells, contract  muscles, transmit nerve impulses and keep the body warm. Animals  get their energy from the large molecules that they eat as food. Glucose  is often the energy source but it may also come from other  carbohydrates, as well as fats and protein. The energy is made by the  biochemical process known as cellular respiration that takes place in  the mitochondria inside every living cell.   The overall reaction can be summarised by the word equation given below.   As you can see from this equation, the cells need to be supplied with oxygen and glucose and the waste product,  carbon dioxide, which is poisonous to cells, needs to be removed. The way the digestive system provides the   glucose for cellular respiration will be described in Chapter 5, but here we are only concerned with the two gases,  oxygen and carbon dioxide, that are involved in cellular respiration. These gases are carried in the blood to and from  the tissues where they are required or produced.   Oxygen enters the body from the air (or water in fish)and carbon dioxide is usually eliminated from the same part of  the body. This process is called gas exchange. In fish gas exchange occurs in the gills, in land dwelling vertebrates  lungs are the gas exchange organs and frogs use gills when they are tadpoles and lungs, the mouth and the skin when  adults.   Mammals (and birds) are active and have relatively high body temperatures so they require large amounts of oxygen  to provide sufficient energy through cellular respiration. In order to take in enough oxygen and release all the carbon  dioxide produced they need a very large surface area over which gas exchange can take place. The many minute air  sacs or alveoli of the lungs provide this. When you look at these under the microscope they appear rather like  bunches of grapes covered with a dense network of fine capillaries (see diagram 9.1). A thin layer of water covers  the inner surface of each alveolus. There is only a very small distance -just 2 layers of thin cells - between the air in  the alveoli and the blood in the capillaries. The gases pass across this gap by diffusion.   The air in the alveoli is rich in oxygen while the blood in the capillaries   around the alveoli is deoxygenated. This is because the haemoglobin in   the red blood cells has released all the oxygen it has been carrying to   the cells of the body. Oxygen diffuses from high concentration to low   alveoli and the capillaries to enter the blood and combine with the   The narrow diameter of the capillaries around the alveoli means that the blood flow is slowed down and that the red  cells are squeezed against the capillary walls. Both of these factors help the oxygen diffuse into the blood (see  diagram 9.2).   When the blood reaches the capillaries of the tissues the oxygen splits from the haemoglobin molecule. It then  diffuses into the tissue fluid and then into the cells.   Blood entering the lung capillaries is full of carbon dioxide that it has collected from the tissues. Most of the carbon  dioxide is dissolved in the plasma either in the form of sodium bicarbonate or carbonic acid. A little is transported  by the red blood cells. As the blood enters the lungs the carbon dioxide gas diffuses through the capillary and alveoli  walls into the water film and then into the alveoli. Finally it is removed from the lungs during breathing out (see  diagram 9.2). (See chapter 8 for more information about how oxygen and carbon dioxide are carried in the blood).   When air is breathed in it passes from the nose to the alveoli of the lungs down a series of tubes (see diagram 9.3).  After entering the nose the air passes through the nasal cavity, which is lined with a moist membrane that adds  warmth and moisture to the air as it passes. The air then flows through the pharynx or throat, a passage that carries  both food and air, to the larynx where the voice-box is located. Here the passages for food and air separate again.  Food must pass into the oesophagus and the air into the windpipe or trachea. To prevent food entering this, a small  flap of tissue called the epiglottis closes the opening during swallowing (see chapter 11). A reflex that inhibits  breathing during swallowing also (usually) prevents choking on food.   The trachea is the tube that ducts the air down the throat. Incomplete rings of cartilage in its walls help keep it open  even when the neck is bent and head turned. The fact that acrobats and people that tie themselves in knots doing  yoga still keep breathing during the most contorted manoeuvres shows how effective this arrangement is. The air  passage now divides into the two bronchi that take the air to the right and left lungs before dividing into smaller and  smaller bronchioles that spread throughout the lungs to carry air to the alveoli. Smooth muscles in the walls of the  bronchi and bronchioles adjust the diameter of the air passages.   The tissue lining the respiratory passages produces mucus and is covered with minute hairs or cilia. Any dust that is  breathed into the respiratory system immediately gets entangled in the mucous and the cilia move it towards the  mouth or nose where it can be coughed up or blown out.   The lungs fill most of the chest or thoracic cavity, which is completely  separated from the abdominal cavity by the diaphragm. The lungs and  the spaces in which they lie (called the pleural cavities) are covered  with membranes called the pleura. There is a thin film of fluid  between the two membranes. This lubricates them as they move over  each other during breathing movements.   The pleural cavities are completely airtight with no connection with the   do this), air rushes in and the lung collapses. Separating the two lungs   is a region of tissue that contains the oesophagus, trachea, aorta, vena cava and lymph nodes. This is called the   mediastinum. In humans and sheep it separates the cavity completely so that puncturing one pleural cavity leads to   the collapse of only one lung. In dogs, however, this separation is incomplete so a puncture results in a complete   The process of breathing moves air in and out of the lungs. Sometimes  this process is called respiration but it is important not to confuse it  with the chemical process, cellular respiration, that takes place in the  mitochondria of cells. Breathing is brought about by the movement of  the diaphragm and the ribs.   The diaphragm is a thin sheet of muscle that completely separates the abdominal and thoracic cavities. When at rest  it domes up into the thoracic cavity but during breathing in or inspiration it flattens. At the same time special  muscles in the chest wall move the ribs forwards and outwards. These movements of both the diaphragm and the ribs  cause the volume of the thorax to increase. Because the pleural cavities are airtight, the lungs expand to fill this  increased space and air is drawn down the trachea into the lungs (see diagram 9.4a).   Expiration or breathing out consists of the opposite movements. The ribs move down and in and the diaphragm  resumes its domed shape so the air is expelled (see diagram 9.4b). Expiration is usually passive and no energy is  required (unless you are blowing up a balloon).   As you sit here reading this just pay attention to your breathing. Notice   you have just rushed here from somewhere else!). Only a small amount   of the total volume that your lungs hold is breathed in and out with   each breath. This kind of gentle "at rest" breathing is called tidal   breathing and the volume breathed in or out (they should be the same)   is the tidal volume (see diagram 9.5). Sometimes people want to   measure the volume of air inspired or expired during a minute of this   normal breathing. This is called the minute volume. It could be   estimated by measuring the volume of one tidal breath and then multiplying that by the number of breaths in a   minute. Of course it is possible to take a deep breath and breathe in as far as you can and then expire as far as   possible. The volume of the air expired when a maximum expiration follows a maximum inspiration is called the   The air animals breathe in consists of 21% oxygen and 0.04% carbon dioxide. Expelled air consists of 16% oxygen  and 4.4% carbon dioxide. This means that the lungs remove only a quarter of the oxygen contained in the air. This is  why it is possible to give someone (or an animal) artificial respiration by blowing expired air into their mouth.   Breathing is usually an unconscious activity that takes place whether you are awake or asleep, although, humans at  least, can also control it consciously. Two regions in the hindbrain called the medulla oblongata and pons control  the rate of breathing. These are called respiratory centres. They respond to the concentration of carbon dioxide in  the blood. When this concentration rises during a bout of activity, for example, nerve impulses are automatically sent  to the diaphragm and rib muscles that increase the rate and the depth of breathing. Increasing the rate of breathing  also increases the amount of oxygen in the blood to meet the needs of this increased activity.   The degree of acidity of the blood (the acid-base balance) is critical for normal functioning of cells and the body as  a whole. For example, blood that is too acidic or alkaline can seriously affect nerve function causing a coma, muscle  spasms, convulsions and even death. Carbon dioxide carried in the blood makes the blood acidic and the higher the  concentration of carbon dioxide the more acidic it is. This is obviously dangerous so there are various mechanisms in  the body that bring the acid-base balance back within the normal range. Breathing is one of these homeostatic  mechanisms. By increasing the rate of breathing the animal increases the amount of dissolved carbon dioxide that is  expelled from the blood. This reduces the acidity of the blood.   Birds have a unique respiratory system that enables them to respire at the very high rates necessary for flight. The  lungs are relatively solid structures that do not change shape and size in the same way as mammalian lungs do.  Tubes run through them and connect with a series of air sacs situated in the thoracic and abdominal body cavities  and some of the bones. Movements of the ribs and breastbone or sternum expand and compress these air sacs so they  act rather like bellows and pump air through the lungs. The evolution of this extremely efficient system of breathing  has enabled birds to migrate vast distances and fly at altitudes higher than the summit of Everest.   • Animals need to breathe to supply the cells with oxygen and remove the waste product carbon dioxide.   • The lungs are situated in the pleural cavities of the thorax.   • Gas exchange occurs in the alveoli of the lungs that provide a large surface area. Here oxygen diffuses from the  alveoli into the red blood cells in the capillaries that surround the alveoli. Carbon dioxide, at high concentration in  the blood, diffuses into the alveoli to be breathed out.   • Inspiration occurs when muscle contraction causes the ribs to move up and out and the diaphragm to flatten.  These movements increase the volume of the pleural cavity and draw air down the respiratory system into the  lungs.   • The air enters the nasal cavity and passes to the pharynx and larynx where the epiglottis closes the opening to  the lungs during swallowing, the air passes down the trachea kept open by rings of cartilage to the bronchi and  bronchioles and then to the alveoli.   • Expiration is a passive process requiring no energy as it relies on the relaxation of the muscles and recoil of the  elastic tissue of the lungs.   • The rate of breathing is determined by the concentration of carbon dioxide in the blood. As carbon dioxide makes  blood acidic, the rate of breathing helps control the acid/base balance of the blood.   • The cells lining the respiratory passages produce mucus which traps dust particles, which are wafted into the nose  by cilia.   Work through the Respiratory System Worksheet to learn the main structures of the respiratory system and how   Then use the Test Yourself below to see how much you remember and understand.   3. What is the process by which oxygen moves from the alveoli into the blood?   5. How does the structure of the alveoli make gas exchange efficient?   7. List the structures that air passes on its way from the nose to the alveoli:   8. What is the function of the mucus and cilia lining the respiratory passages?   9. How do movements of the ribs and diaphragm bring about inspiration? Circle the correct statement below.   a) The diaphragm domes up into the thorax and ribs move in and down   c) The diaphragm domes up into the thorax and the ribs move up and out.   d) The diaphragm flattens and the ribs move in and down   A good interactive explanation of breathing and gas exchange in humans with diagrams to label, animations to watch  and questions to answer.   Although this is of the human respiratory system there is a good diagram that gives the functions of the various parts  as you move your mouse over it. Also an animation of gas exchange and a quiz to test your understanding of it.   Wikipedia on the lungs. Lots of good information on the human respiratory system with all sorts of links if you are  interested.   Gas exchange is the addition of oxygen to the blood and the removal of carbon dioxide.   3. What is the process by which oxygen moves from the alveoli into the blood?   This process occurs because oxygen is at a much higher concentration in the alveoli than in the blood.  Diffusion always occurs from a high to low concentration.   5. How does the structure of the alveoli make gas exchange efficient?   The thin walls of the alveoli and the capillaries give only a small distance for the oxygen and carbon dioxide  to diffuse across making gas exchange efficient.   Oxygen is carried in the blood combined with haemoglobin in the red blood cells.   7. List the structures that air passes on its way from the nose to the alveoli:   On its way to the alveoli of the lungs air travels through the nasal cavity, pharynx, larynx, trachea, bronchi  and bronchioles.   8. What is the function of the mucous and cilia lining the respiratory passages?   The mucus and cilia in the respiratory passages trap dust particles and transport them to the mouth and nose  for expulsion.   9. How do movements of the ribs and diaphragm bring about inspiration? Circle the correct statement below.   The epiglottis closes the larynx during swallowing so that food cannot enter the trachea.   The concentration of carbon dioxide in the blood controls the rate of breathing.   • the basic structure and function of a lymph node  and the position of some important lymph nodes  in the body   • the route by which lymph circulates in the body  and is returned to the blood system   • the location and function of the spleen, thymus  and lac teals   following the absorption of fat from the digested food into the lymphatics (lacteals) of the villi (see chapter 1 1 for   more on these). From the lymph capillaries the lymph flows into larger tubes called lymphatic vessels. These carry   the lymph back to join the blood circulation (see diagrams 10.1 and 10.2).   lymphatic vessel  artery carrying ' * * "" "*"* ~ "" "" "" * '" '   Lymphatic vessels have several similarities to veins. Both are thin walled and return fluid to the right hand side of  the heart. The movement of the fluid in both is brought about by the contraction of the muscles that surround them  and both have valves to prevent backflow. One important difference is that lymph passes through at least one lymph  node or gland before it reaches the blood system (see diagram 10.2). These filter out used cell parts, cancer cells and  bacteria and help defend the body from infection.   Lymph nodes are of various sizes and shapes and found throughout the body and the more important ones are shown  in diagram 10.3. They consist of lymph tissue surrounded by a fibrous sheath. Lymph flows into them through a  number of incoming vessels. It then trickles through small channels where white cells called macrophages (derived  from monocytes) remove the bacteria and debris by engulfing and digesting them (see diagram 10.4). The lymph  then leaves the lymph nodes through outgoing vessels to continue its journey towards the heart where it rejoins the  blood circulation (see diagrams 10.2 and 10.3).   foreign substances  filtered out of lymph  as it trickles through  lymph node   As well as filtering the lymph, lymph nodes produce the white cells known as lymphocytes. Lymphocytes are also  produced by the thymus, spleen and bone marrow. There are two kinds of lymphocyte. The first attach invading  micro organisms directly while others produce antibodies that circulate in the blood and attack them.   The function of the lymphatic system can therefore be summarized as transport and defense. It is important for  returning the fluid and proteins that have escaped from the blood capillaries to the blood system and is also  responsible for picking up the products of fat digestion in the small intestine. Its other essential function is as part of  the immune system, defending the body against infection.   During infection of the body the lymph nodes often become swollen and tender because of their increased activity.  This is what causes the swollen 'glands' in your neck during throat infections, mumps and tonsillitis. Sometimes the  bacteria multiply in the lymph node and cause inflammation. Cancer cells may also be carried to the lymph nodes  and then transported to other parts of the body where they may multiply to form a secondary growth or metastasis.  The lymphatic system may therefore contribute to the spread of cancer. Inactivity of the muscles surrounding the  lymphatic vessels or blockage of these vessels causes tissue fluid to 'back up' in the tissues resulting in swelling or  oedema.   The spleen is an important part of the lymphatic system. It is a deep red organ situated in the abdomen caudal to the  stomach (see diagram 10.3). It is composed of two different types of tissue. The first type makes and stores  lymphocytes, the cells of the immune system. The second type of tissue destroys worn out red blood cells, breaking  down the haemoglobin into iron, which is recycled, and waste products that are excreted. The spleen also stores red  blood cells. When severe blood loss occurs, it contracts and releases them into the circulation.   The thymus is a large pink organ lying just under the sternum (breastbone) just cranial to the heart (see diagram  10.1). It has an important function processing lymphocytes so they are capable of recognising and attacking foreign  invaders like bacteria.   Other lymph organs are the bone marrow of the long bones where lymphocytes are produced and lymph nodules,  which are like tiny lymph nodes. Large clusters of these are found in the wall of the small intestine (called Peyer's  Patches) and in the tonsils.   • Fluid leaks out of the thin walled capillaries as they pass through the tissues. This is called tissue fluid.   • Much of tissue fluid passes back into the capillaries. Some enters the blind-ended lymphatic capillaries that form  a network between the cells of the tissues. This fluid is called lymph.   • Lymph flows from the lymphatic capillaries to lymph vessels, passing through lymph nodes and along the  thoracic duct to join the blood system.   • Other organs of the lymphatic system are the spleen, thymus, bone marrow, and lymph nodules.   2. By what route does lymph make its way back to join the blood of the circulatory system?   3. As the lymphatic system has no heart to push the lymph along what makes it flow?   4. What happens to the lymph as it passes through a lymph node?   A nice clear explanation here with great diagrams of the (human) lymphatic system.   Introduction to the Lymphatic System. A good description of lymph circulation with an animation.   Good information here on the (human) lymphatic system, lymph circulation and lymphoid organs.   1. What is the difference between tissue fluid and lymph? Tissue fluid is the fluid that has leaked out of the  capillaries into the spaces around the cells of the tissues. Lymph is tissue fluid that has entered the lymphatic system  i.e. the difference is largely one of location.   2. By what route does lymph make its way back to join the blood of the circulatory system? Lymph makes it way  back to join the blood in the circulatory system via the lymphatic capillaries, lymphatic vessels, at least one lymph  node and the thoracic duct. The lymph then joins the blood in veins that carry it back to the heart.   3. As the lymphatic system has no heart to push the lymph along what makes it flow? As the lymphatic system has no  heart to push the lymph along it is pushed along mainly by the contraction of skeletal muscles that lie alongside the  lymphatics.   4. What happens to the lymph as it passes through a lymph node? As the lymph passes through a lymph node,  bacteria and debris are filtered from it and lymphocytes are added to it.   5. Where is the spleen located in the body? The spleen is found in the abdomen alongside and slightly caudal to the  stomach.   6. Where is the thymus located in the body? The thymus gland is located in the thorax beneath the sternum and just  cranial to the heart.   7. What is the function of lymphocytes? Lymphocytes defend the body from invasion by microorganisms. Some  attach to the organisms directly while others produce antibodies that circulate in the blood and attack them.   • what is meant by the terms: ingestion, digestion,  absorption, assimilation, egestion, peristalsis and  chyme   • the characteristics, advantages and disadvantages  of a herbivorous, carnivorous and omnivorous diet   • the parts of the gut in the order in which the food  passes down it   Plant cells are made of organic molecules using  energy from the sun. This process is called  photosynthesis. Animals rely on these ready-made  organic molecules to supply them with their food.  Some animals (herbivores) eat plants; some  (carnivores) eat the herbivores.   break down the large cellulose molecules in the plant cell walls. Micro-organisms like bacteria, on the other hand,   can break them down. Therefore herbivores employ micro-organisms to do the job for them.   The first, ruminants like cattle, sheep and goats, house these bacteria in a special compartment in the enlarged  stomach called the rumen.   The second group has an enlarged large intestine and caecum, called a functional caecum, occupied   by cellulose digesting micro-organisms. These non-ruminant herbivores include the horse, rabbit and rat.   Even with the help of microorganisms, eating plants still poses problems. Plants are not a good source of nutrients,  and herbivores have to eat large quantities of food to obtain all they require. Herbivores like cows, horses and rabbits  typically spend much of their day feeding. To give the micro- organisms access to the cellulose molecules, the plant  cell walls need to be broken down. This is why herbivores have teeth that are adapted to crush and grind. Their guts  also tend to be lengthy and the food takes a long time to pass through it.   Eating plants does have some advantages. Plants are immobile so herbivores normally have to spend little energy  collecting them. This contrasts with the other main group of animals- the carnivores that often have to chase their  prey.   Carnivorous animals like those in the cat and dog families, bears, seals, crocodiles and birds of prey catch and eat  other animals. They often have to use large amounts of energy finding, stalking, catching and killing their prey.  However, they are rewarded by the fact that meat provides a very concentrated source of nutrients. Carnivores in the  wild therefore tend to eat distinct meals often with long and irregular intervals between them. Time after feeding is  spent digesting and absorbing the food.   The guts of carnivores are usually shorter and less complex than those of herbivores because meat is easier to digest  than plant material. Carnivores usually have teeth that are specialised for dealing with flesh, gristle and bone. They  have sleek bodies, strong, sharp claws and keen senses of smell, hearing and sight. They are also often cunning, alert  and have an aggressive nature.   Many animals feed on both animal and vegetable material — they are omnivorous. Most primates including humans  belong to this category as do pigs and rats. Their food is diverse, ranging from plant material to animals they have  either killed themselves or scavenged from other carnivores. Omnivores lack the specialised teeth and guts of  carnivores and herbivores but are often highly intelligent and adaptable reflecting their varied diet.   Whether an animal eats plants or flesh, the carbohydrates, fats and proteins in the food it eats are generally giant  molecules (see chapter 1). These need to be split up into smaller ones before they can pass into the blood and enter  the cells to be used for energy or to make new cell constituents.   Carbohydrates like cellulose, starch, and glycogen need to be split into glucose and other monosaccharides;   Fats or lipids need to be split into fatty acids and glycerol.   The digestive tract, alimentary canal or gut is a hollow tube stretching from the mouth to the anus. It is the organ  system concerned with the treatment of foods.   At the mouth the large food molecules are taken into the gut - this is called ingestion. They must then be broken  down into smaller ones by digestive enzymes - digestion, before they can be taken from the gut into the blood  stream - absorption. The cells of the body can then use these small molecules - assimilation. The indigestible waste  products are eliminated from the body by the act of egestion (see diagram 11.1).   Diagram 11.1- From ingestion to egestion  The 4 major functions of the gut are:   2. Processing the food physically by breaking it up (chewing), mixing, adding fluid etc.   3. Processing the food chemically by adding digestive enzymes to split large food molecules into smaller ones.   4. Absorbing these small molecules into the blood stream so the body can use them.  The regions of a typical mammals gut (for example a cat or dog) are shown in diagram 11.2.   The food that enters the mouth passes to the oesophagus, then to the stomach, small intestine, cecum, large  intestine, rectum and finally undigested material exits at the anus. The liver and pancreas produce secretions that  aid digestion and the gall bladder stores bile. Animals do not have an appendix.   The mouth takes food into the body. The lips hold the food inside the mouth during chewing and allow the baby  animal to suck on its mother's teat. In elephants the lips (and nose) have developed into the trunk which is the main  food collecting tool. Some mammals, e.g. hamsters, have stretchy cheek pouches that they use to carry food or  material to make their nests.   The sight or smell of food and its presence in the mouth stimulates the salivary glands to secrete saliva. There are  four pairs of these glands in cats and dogs (see diagram 11.3). The fluid they produce moistens and softens the food  making it easier to swallow. It also contains the enzyme, salivary amylase, which starts the digestion of starch.   The tongue moves food around the mouth and rolls it into a ball for swallowing. Taste buds are located on the  tongue and in dogs and cats it is covered with spiny projections used for grooming and lapping. The cow's tongue is  prehensile and wraps around grass to graze it.   Swallowing is a complex reflex involving 25 different muscles. It pushes food into the oesophagus and at the same  time a small flap of tissue called the epiglottis closes off the windpipe so food doesn't go 'down the wrong way' and  choke the animal (see diagram 1 1.4).   Diagram 1 1 .4 - Section through the head of a dog   Teeth seize, tear and grind food. They are inserted into sockets in the bone and consist of a crown above the gum and  root below. The crown is covered with a layer of enamel, the hardest substance in the body. Below this is the  dentine, a softer but tough and shock resistant material. At the centre of the tooth is a space filled with pulp which  contains blood vessels and nerves. The tooth is cemented into the socket and in most teeth the tip of the root is quite  narrow with a small opening for the blood vessels and nerves (see diagram 1 1.5).   In teeth that grow continuously, like the incisors of rodents, the opening remains large and these teeth are called  open rooted teeth. Mammals have 2 distinct sets of teeth. The first the milk teeth are replaced by the permanent  teeth.   All the teeth of fish and reptiles are similar but mammals usually have four different types of teeth.   The incisors are the chisel-shaped 'biting off teeth at the front of the mouth. In rodents and rabbits the incisors never  stop growing (open-rooted teeth). They must be worn or ground down continuously by gnawing. They have hard  enamel on one surface only so they wear unevenly and maintain their sharp cutting edge.   The largest incisors in the animal kingdom are found in elephants, for tusks are actually giant incisors. Sloths have  no incisors at all, and sheep have no incisors in the upper jaw (see diagram 1 1.6). Instead there is a horny pad against  which the bottom incisors cut.   The canines or 'wolf-teeth' are long, cone-shaped teeth situated just behind the incisors. They are particularly well  developed in the dog and cat families where they are used to hold, stab and kill the prey (see diagram 1 1.7).   The tusks of boars and walruses are large canines while rodents and herbivores like sheep have no (or reduced)  canines. In these animals the space where the canines would normally be is called the diastema. In rodents like the  rat and beaver it allows the debris from gnawing to be expelled easily.   The cheek teeth or premolars and molars crush and grind the food. They are particularly well developed in  herbivores where they have complex ridges that form broad grinding surfaces (see diagram 11.6). These are created  from alternating bands of hard enamel and softer dentine that wear at different rates.   In carnivores the premolars and molars slice against each other like scissors and are called carnassial teeth see  diagram 1 1.7). They are used for shearing flesh and bone.   The numbers of the different kinds of teeth can be expressed in a dental formula. This gives the numbers of  incisors, canines, premolars and molars in one half of the mouth. The numbers of these four types of teeth in the left  or right half of the upper jaw are written above a horizontal line and the four types of teeth in the right or left half  of the lower jaw are written below it.   It indicates that in the upper right (or left) half of the jaw there are no incisors or canines (i.e. there is a diastema),  three premolars and three molars. In the lower right (or left) half of the jaw are three incisors, one canine, three  premolars and three molars (see diagram 11.6).   The formula indicates that in the right (or left) half of the upper jaw there are three incisors, one canine, four  premolars and two molars. In the right (or left) half of the lower jaw there are three incisors, one canine, four  premolars and three molars (see diagram 11.7).   The oesophagus transports food to the stomach. Food is moved along the oesophagus, as it is along the small and  large intestines, by contraction of the smooth muscles in the walls that push the food along rather like toothpaste  along a tube. This movement is called peristalsis (see diagram 1 1.8).   The stomach stores and mixes the food. Glands in the wall secrete gastric juice that contains enzymes to digest  protein and fats as well as hydrochloric acid to make the contents very acidic. The walls of the stomach are very  muscular and churn and mix the food with the gastric juice to form a watery mixture called chyme (pronounced  kime). Rings of muscle called sphincters at the entrance and exit to the stomach control the movement of food into  and out of it (see diagram 11.9).   Most of the breakdown of the large food molecules and absorption of the smaller molecules take place in the long  and narrow small intestine. The total length varies but it is about 6.5 metres in humans, 21 metres in the horse, 40  metres in the ox and over 150 metres in the blue whale.   It is divided into 3 sections: the duodenum (after the stomach), jejunum and ileum. The duodenum receives 3  different secretions:   These complete the digestion of starch, fats and protein. The products of digestion are absorbed into the blood and  lymphatic system through the wall of the intestine, which is lined with tiny finger-like projections called villi that  increase the surface area for more efficient absorption (see diagram 11.10).   Diagram 11.10 - The wall of the small intestine showing villi   In ruminant herbivores like cows, sheep and antelopes the stomach is highly modified to act as a "fermentation vat".  It is divided into four parts. The largest part is called the rumen. In the cow it occupies the entire left half of the  abdominal cavity and can hold up to 270 litres. The reticulum is much smaller and has a honeycomb of raised folds  on its inner surface. In the camel the reticulum is further modified to store water. The next part is called the omasum  with a folded inner surface. Camels have no omasum. The final compartment is called the abomasum. This is the  'true' stomach where muscular walls churn the food and gastric juice is secreted (see diagram 11.11).   Ruminants swallow the grass they graze almost without chewing and it passes down the oesophagus to the rumen  and reticulum. Here liquid is added and the muscular walls churn the food. These chambers provide the main  fermentation vat of the ruminant stomach. Here bacteria and single-celled animals start to act on the cellulose plant  cell walls. These organisms break down the cellulose to smaller molecules that are absorbed to provide the cow or  sheep with energy. In the process, the gases methane and carbon dioxide are produced. These cause the "burps" you  may hear cows and sheep making.   Not only do the micro-organisms break down the cellulose but they also produce the vitamins E, B and K for use by  the animal. Their digested bodies provide the ruminant with the majority of its protein requirements.   In the wild grazing is a dangerous activity as it exposes the herbivore to predators. They crop the grass as quickly as  possible and then when the animal is in a safer place the food in the rumen can be regurgitated to be chewed at the  animal's leisure. This is 'chewing the cud' or rumination. The finely ground food may be returned to the rumen for  further work by the microorganisms or, if the particles are small enough, it will pass down a special groove in the  wall of the oesophagus straight into the omasum. Here the contents are kneaded and water is absorbed before they  pass to the abomasum. The abomasum acts as a "proper" stomach and gastric juice is secreted to digest the protein.   The large intestine consists of the caecum, colon and rectum. The chyme from the small intestine that enters the  colon consists mainly of water and undigested material such as cellulose (fibre or roughage). In omnivores like the  pig and humans the main function of the colon is absorption of water to give solid faeces. Bacteria in this part of the  gut produce vitamins B and K.   The caecum, which forms a dead-end pouch where the small intestine joins the large intestine, is small in pigs and  humans and helps water absorption. However, in rabbits, rodents and horses, the caecum is very large and called the  functional caecum. It is here that cellulose is digested by micro-organisms. The appendix, a narrow dead end tube  at the end of the caecum, is particularly large in primates but seems to have no digestive function.   The caecum in the rabbit, rat and guinea pig is greatly enlarged to provide a "fermentation vat" for micro-organisms  to break down the cellulose plant cell walls. This is called a functional caecum (see diagram 11.12). In the horse  both the caecum and the colon are enlarged. As in the rumen, the large cellulose molecules are broken down to  smaller molecules that can be absorbed. However, the position of the functional caecum after the main areas of  digestion and absorption, means it is potentially less effective than the rumen. This means that the small molecules  that are produced there can not be absorbed by the gut but pass out in the faeces. The rabbit and rodents (and foals)  solve this problem by eating their own faeces so that they pass through the gut a second time and the products of  cellulose digestion can be absorbed in the small intestine. Rabbits produce two kinds of faeces. Softer night-time  faeces are eaten directly from the anus and the harder pellets you are probably familiar with, that have passed  through the gut twice.   Birds' guts have important differences from mammals' guts. Most obviously, birds have a beak instead of teeth.  Beaks are much lighter than teeth and are an adaptation for flight. Imagine a bird trying to take off and fly with a  whole set of teeth in its head! At the base of the oesophagus birds have a bag-like structure called a crop. In many  birds the crop stores food before it enters the stomach, while in pigeons and doves glands in the crop secretes a  special fluid called crop-milk which parent birds regurgitate to feed their young. The stomach is also modified and  consists of two compartments. The first is the true stomach with muscular walls and enzyme secreting glands. The  second compartment is the gizzard. In seed eating birds this has very muscular walls and contains pebbles  swallowed by the bird to help grind the food. This is the reason why you must always supply a caged bird with grit.  In birds of prey like the falcon the walls of the gizzard are much thinner and expand to accommodate large meals  (see diagram 11.13).   Diagram 11.13 - The stomach and small intestine of a hen   During digestion the large food molecules are broken down into smaller molecules by enzymes. The three most  important groups of enzymes secreted into the gut are:   1. Amylases that split carbohydrates like starch and glycogen into monosaccharides like glucose.   3. Lipases that split lipids or fats into fatty acids and glycerol.   Glands produce various secretions which mix with the food as it passes along the gut.  These secretions include:   1. Saliva secreted into the mouth from several pairs of salivary glands (see diagram 11.3). Saliva consists  mainly of water but contains salts, mucous and salivary amylase. The function of saliva is to lubricate food as  it is chewed and swallowed and salivary amylase begins the digestion of starch.   2. Gastric juice secreted into the stomach from glands in its walls. Gastric juice contains pepsin that breaks  down protein and hydrochloric acid to produce the acidic conditions under which this enzyme works best. In  baby animals rennin to digest milk is also produced in the stomach.   3. Bile produced by the liver. It is stored in the gall bladder and secreted into the duodenum via the bile duct  (see diagram 1 1.14). (Note that the horse, deer, parrot and rat have no gall bladder). Bile is not a digestive  enzyme. Its function is to break up large globules of fat into smaller ones so the fat splitting enzymes can gain  access the fat molecules.   The pancreas is a gland located near the beginning of the duodenum (see diagram 1 1.14). In most animals it is large  and easily seen but in rodents and rabbits it lies within the membrane linking the loops of the intestine (the  mesentery) and is quite difficult to find. Pancreatic juice is produced in the pancreas. It flows into the duodenum  and contains amylase for digesting starch, lipase for digesting fats and protease for digesting proteins.   Intestinal juice is produced by glands in the lining of the small intestine. It contains enzymes for digesting  disaccharides and proteins as well as mucus and salts to make the contents of the small intestine more alkaline so the  enzymes can work.   The small molecules produced by digestion are absorbed into the villi of the wall of the small intestine. The tiny  finger-like projections of the villi increase the surface area for absorption. Glucose and amino acids pass directly  through the wall into the blood stream by diffusion or active transport. Fatty acids and glycerol enter vessels of the  lymphatic system (lacteals) that run up the centre of each villus.   The liver is situated in the abdominal cavity adjacent to the diaphragm (see diagrams 2 and 14). It is the largest  single organ of the body and has over 100 known functions. Its most important digestive functions are:   1 . the production of bile to help the digestion of fats (described above) and   Glucose is absorbed into the capillaries of the villi of the intestine. The blood stream takes it directly to the liver via a  blood vessel known as the hepatic portal vessel or vein (see diagram 11.15).   The liver converts this glucose into glycogen which it stores. When glucose levels are low the liver can convert the  glycogen back into glucose. It releases this back into the blood to keep the level of glucose constant. The hormone  insulin, produced by special cells in the pancreas, controls this process.   Diagram 11.15 - The control of blood glucose by the liver  Other functions of the liver include:   4. making the proteins that are found in the blood plasma (albumin, globulin and fibrinogen),   6. removing toxic substances like alcohol and poisons from the blood and converting them to safer  substances,   7. producing heat to help maintain the temperature of the body.    Diagram 11.16- Summary of the main functions of the different regions of the gut   • The gut breaks down plant and animal materials into nutrients that can be used by animals' bodies.   • Plant material is more difficult to break down than animal tissue. The gut of herbivores is therefore longer and  more complex than that of carnivores. Herbivores usually have a compartment (the rumen or functional  caecum) housing micro-organisms to break down the cellulose wall of plants.   • Chewing by the teeth begins the food processing. There are 4 main types of teeth: incisors, canines, premolars  and molars. In dogs and cats the premolars and molars are adapted to slice against each other and are called  carnassial teeth.   • Saliva is secreted in the mouth. It lubricates the food for swallowing and contains an enzyme to break down  starch.   • Chewed food is swallowed and passes down the oesophagus by waves of contraction of the wall called  peristalsis. The food passes to the stomach where it is churned and mixed with acidic gastric juice that begins  the digestion of protein.   • The resulting chyme passes down the small intestine where enzymes that digest fats, proteins and carbohydrates  are secreted. Bile produced by the liver is also secreted here. It helps in the breakdown of fats. Villi provide the  large surface area necessary for the absorption of the products of digestion.   • In the colon and caecum water is absorbed and micro organisms produce some vitamin B and K. In rabbits,  horses and rodents the caecum is enlarged as a functional caecum and micro-organisms break down cellulose  cell walls to simpler carbohydrates. Waste products exit the body via the rectum and anus.   • The pancreas produces pancreatic juice that contains many of the enzymes secreted into the small intestine.   • In addition to producing bile the liver regulates blood sugar levels by converting glucose absorbed by the villi into  glycogen and storing it. The liver also removes toxic substances from the blood, stores iron, makes vitamin A and  produces heat.   Use the Digestive System Worksheet to help you learn the different parts of the digestive system and their  functions.   Then work through the Test Yourself below to see if you have understood and remembered what you learned.   2. Give 2 facts about how the teeth of cats and dogs are adapted for a carnivorous diet:   11. How does the caecum of rabbits differ from that of cats?   12. How does the liver help control the glucose levels in the blood?   • http://www.second-opinions.co.uk/carn_herb_comparison.html Second opinion. A good comparison of the  guts of carnivores and herbivores   • http://www.chu.cam.ac.uk/~ALRF/giintro.htm The gastrointestinal system. A good comparison of the guts  of carnivores and herbivores with more advanced information than in the previous site.   • http://en.wikipedia.org/wiki/Digestion Wikipedia on digestion with links to further information on most  aspects. Like most websites this is mainly about human digestion but much is applicable to animals.   The four different kinds of teeth are incisors, canines, premolars and molars.   2. Give 2 facts about how the teeth of cats and dogs are adapted for a carnivorous diet   The teeth of cats and dogs are adapted for a carnivorous diet with small chisel shaped incisors for biting off  flesh, sharp canines for holding and killing prey and carnassials for shearing flesh.   Saliva lubricates food for swallowing and contains an enzyme for breaking down starch.   Peristalsis is the waves of smooth muscle contraction that pass down the gut and move the food along.   In the stomach food is churned and mixed and gastric juice that contains enzymes to digest proteins is added.  Hydrochloric acid to make the stomach contents acidic for the protein digesting enzymes is also added here.   Chyme is the liquefied and partially digested food that passes down the gut.   Villi are the small finger-like projections from the wall of the small intestine that increase the surface area for  the absorption into the blood of the digested nutrients.   In the small intestine food is digested and resulting nutrients are absorbed into the blood via the villi.   The pancreas is situated in the first bend of the duodenum after it leaves the stomach. It produces pancreatic  juice that contains enzymes for digesting carbohydrates, fats and proteins.   11. How does the caecum of rabbits differ from that of cats?   In rabbits the caecum is enlarged to form a functional caecum that contains microorganisms for breaking  down the cellulose walls of plants. Note that because of the position of the functional caecum at the end of the  gut animals like the rabbit eat their faeces to take advantage of the digested nutrients.   12. How does the liver help control the glucose levels in the blood?   The liver helps control the glucose levels in the blood by converting the glucose to glycogen that it stores. This  is then released when blood glucose levels are low. The hormone insulin produced by the pancreas controls  this process.   Other functions of the liver include the detoxification of toxic substances like alcohol, storing iron and  production of bile, proteins, vitamin A and heat.   • the structure and function of a kidney tubule or  nephron   • the processes of filtration, reabsorption, secretion  and concentration that convert blood to urine in  the kidney tubule   • the functions of the kidney in excreting  nitrogenous waste, controlling water levels and  regulating salt concentrations and acid-base  balance   The cells of an animal can only remain healthy if the   conditions are just right. The processes that take place in them are upset if the temperature is too high or too low, or  if the fluid around or inside them is too acid or alkaline. Homeostasis is the name given to the processes that help  keep the internal conditions constant even when external conditions change. The word means, "staying the same".   There are a number of organs in the body that play a part in maintaining homeostasis. For example, the skin helps  keep the internal temperature of bird and mammals bodies within a narrow range even when the outside temperatures  change (see Chapters 5 and 16); the lungs control the amount of carbon dioxide in the blood (see Chapters 8 and 16);  the liver and pancreas work together to keep the amount of glucose in the blood within narrow limits (see Chapter 5)  and the kidneys regulate the acidity and the concentration of water and salt in the blood (also see Chapter 16). How  the kidneys do this will be described later in this chapter.   Hormones are chemicals that carry messages around the body in the blood and are central to many of the  homeostatic processes mentioned above. Their role will be described in more detail in Chapter 16.   Water is essential for living things to survive because all the chemical reactions within a body take place in a  solution of water. An animal's body consists of up to 80% water. The exact proportion depends on the type of  animal, its age, sex, health and whether or not it has had sufficient to drink. Generally animals do not survive a loss  of more than 15% of their body water.   In vertebrates almost 2/3rd of this water is in the cells (intracellular fluid). The rest is outside the cells  (extracellular fluid) where it is found in the spaces around the cells (tissue fluid), as well as in the blood and  lymph.   Animals lose water through their skin and lungs, in the faeces and urine. These losses must be made up by water in  food and drink and from the water that is a by-product of chemical reactions. If the animal does not manage to  compensate for water loss the dissolved substances in the blood may become so concentrated they become lethal. To  prevent this happening various mechanisms come into play as soon as the concentration of the blood increases. A  part of the brain called the hypothalamus is in charge of these homeostatic processes. The most important is the  feeling of thirst that is triggered by an increase in blood concentration. This stimulates an animal to find water and  drink it.   The kidneys are also involved in maintaining water balance as various hormones instruct them to produce more  concentrated urine and so retain some of the water that would otherwise be lost (see later in this Chapter and Chapter  16).   Coping with water loss is a particular problem for animals that live in dry conditions. Some, like the camel, have  developed great tolerance for dehydration. For example, under some conditions, camels can withstand the loss of one  third of their body mass as water. They can also survive wide daily changes in temperature. This means they do not  have to use large quantities of water in sweat to cool the body by evaporation.   Smaller animals are more able than large ones to avoid extremes of temperature or dry conditions by resting in  sheltered more humid situations during the day and being active only at night.   The kangaroo rat is able to survive without access to any drinking water at all because it does not sweat and produces  extremely concentrated urine. Water from its food and from chemical processes is sufficient to supply all its  requirements.   Animals need to excrete because they take in substances that are excess to the body's requirements and many of the  chemical reactions in the body produce waste products. If these substances were not removed they would poison  cells or slow down metabolism. All animals therefore have some means of getting rid of these wastes.   The major waste products in mammals are carbon dioxide that is removed by the lungs, and urea that is produced  when excess amino acids (from proteins) are broken down. Urea is filtered from the blood by the kidneys.    Diagram 12.2 - The position of the organs of the urinary system in a dog   The kidneys in mammals are bean-shaped organs that lie in the abdominal cavity attached to the dorsal wall on  either side of the spine (see diagram 12.2). An artery from the dorsal aorta called the renal artery supplies blood to  them and the renal vein drains them.   To the naked eye kidneys seem simple enough organs. They are covered by a fibrous coat or capsule and if cut in  half lengthways (longitudinally) two distinct regions can be seen - an inner region or medulla and the outer cortex.  A cavity within the kidney called the pelvis collects the urine and carries it to the ureter, which connects with the  bladder where the urine is stored temporarily. Rings of muscle (sphincters) control the release of urine from the  bladder and the urine leaves the body through the urethra (see diagrams 12.3 and 12.4).   It is only when you examine kidneys under the microscope that you find that their structure is not simple at all. The  cortex and medulla are seen to be composed of masses of tiny tubes. These are called kidney tubules or nephrons  (see diagrams 12.5 and 12.6). A human kidney consists of over a million of them.   looped portion of tubule  (Loop of Henle) surrounded  by blood capillaries   At one end of each nephron, in the cortex of the kidney, is a cup shaped structure called the (Bowman's or renal)  capsule. It surrounds a tuft of capillaries called the glomerulus that carries high-pressure blood. Together the  glomerulus and capsule act as a blood-filtering device (see diagram 12.7). The holes in the filter allow most of the  contents of the blood through except the red and white cells and large protein molecules . The fluid flowing from the  capsule into the rest of the kidney tubule is therefore very similar to blood plasma and contains many useful  substances like water, glucose, salt and amino acids. It also contains waste products like urea.   After entering the glomerulus the filtered fluid flows along a coiled part of the tubule (the proximal convoluted  tubule) to a looped portion (the Loop of Henle) and then to the collecting tube via a second length of coiled tube  (the distal convoluted tubule) (see diagram 12.6). From the collecting ducts the urine flows into the renal pelvis  and enters the ureter.   Note that the glomerulus, capsule and both coiled parts of the tubule are all situated in the cortex of the kidney while  the loops of Henle and collecting ducts make up the medulla (see diagram 12.5).   As the fluid flows along the proximal convoluted tubule useful substances like glucose, water, salts, potassium ions,  calcium ions and amino acids are reabsorbed into the blood capillaries that form a network around the tubules.  Many of these substances are transported by active transport and energy is required.   In a separate process, some substances, particularly potassium, ammonium and hydrogen ions, and drugs like  penicillin, are actively secreted into the distal convoluted tubule.   By the time the fluid has reached the collecting ducts these processes of absorption and secretion have changed the  fluid originally filtered into the Bowman's capsule into urine. The main function of the collecting ducts is then to  remove more water from the urine if necessary. These processes are summarised in diagram 12.8.   Normal urine consists of water, in which waste products such as urea and salts such as sodium chloride are  dissolved. Pigments from the breakdown of red blood cells give urine its yellow colour.   Because of the high pressure of the blood in the glomerulus and the large size of the pores in the  glomerulus/capsule-filtering device, an enormous volume of fluid passes into the kidney tubules. If this fluid were  left as it is, the animal's body would be drained dry in 30 minutes. In fact, as the fluid flows down the tubule, over  90% of the water in it is reabsorbed. The main part of this reabsorption takes place in the collecting tubes.   The amount of water removed from the collecting ducts is controlled by a hormone called antidiuretic hormone  (ADH) produced by the pituitary gland, situated at the base of the brain. When the blood becomes more  concentrated, as happens when an animal is deprived of water, ADH is secreted and causes more water to be  absorbed from the collecting ducts so that concentrated urine is produced. When the animal has drunk plenty of  water and the blood is dilute, no ADH is secreted and no or little water is absorbed from the collecting ducts, so  dilute urine is produced. In this way the concentration of the blood is controlled precisely.   Diagram 12.8 - Summary of the processes involved in the formation of urine   Although the skin of fish is more or less waterproof, the gills are very porous. The body fluids of fish that live in  fresh water have a higher concentration of dissolved substances than the water in which they swim. In other words  the body fluids of fresh water fish are hypertonic to the water (see chapter 3). Water therefore flows into the body  by osmosis. To stop the body fluids being constantly diluted fresh water fish produce large quantities of dilute urine.   Marine fish like the sharks and dogfish have body fluids that have the same concentration of dissolved substances as  the water (isotonic) have little problem with water balance. However, marine bony fish like red cod, snapper and  sole, have body fluids with a lower concentration of dissolved substances than seawater (they are hypotonic to  seawater). This means that water tends to flow out of their bodies by osmosis. To make up this fluid loss they drink  seawater and get rid of the excess salt by excreting it from the gills.   Marine birds that eat marine fish take in large quantities of salt and some only have access to seawater for drinking.  Bird's kidneys are unable to produce very concentrated urine, so they have developed a salt gland. This excretes a  concentrated salt solution into the nose to get rid of the excess salt.   There are two types of diabetes. The most common is called sugar diabetes or diabetes mellitus and is common in  cats and dogs especially if they are overweight. It is caused by the pancreas secreting insufficient insulin, the  hormone that controls the amount of glucose in the blood. If insulin secretion is inadequate, the concentration of  glucose in the blood increases. Any increase in the glucose in the blood automatically leads to an increase in glucose  in the fluid filtered into the kidney tubule. Normally the kidney removes all the glucose filtered into it, but these high  concentrations swamp this removal mechanism and urine containing glucose is produced. The main symptoms of  this type of diabetes are the production of large amounts of dilute urine containing glucose, and excessive thirst.   The second type of diabetes is called diabetes insipidus. The name comes from the main symptom, which is the  production of large amounts of very dilute and "tasteless" urine. It occurs when the pituitary gland produces  insufficient ADH, the hormone that stimulates water re-absorption from the kidney tubule. When this hormone is  lacking, water is not absorbed and large amounts of dilute urine are produced. Because so much water is lost in the  urine, animals with this form of diabetes can die if deprived of water for only a day or so.   The excretion of urea from the body and the maintenance of water balance, as described above, are the main  functions of the kidney. However, the kidneys have other roles in keeping conditions in the body stable i.e. in  maintaining homeostasis. These include:   • controlling the concentration of salt ions (Na+, K+, C1-) in the blood by adjusting how much is excreted or  retained;   • maintaining the correct acidity of the blood. Excess acid is constantly being produced by the normal chemical  reactions in the body and the kidney eliminates this.   Normal urine consists of water (95%), urea, salts (mostly sodium chloride) and pigments (mostly from bile) that give  it its characteristic colour.   If the body is not working properly, small amounts of substances not normally present may be found in the urine or  substances normally present may appear in abnormal amounts.   • Urine with red blood cells in it is called haematuria, and may indicate inflammation of the kidney ,or urinary  tract, cancer or a blow to the kidneys.   • Sometimes free haemoglobin is found in the urine. This indicates that the red blood cells in the blood have  haemolysed (the membrane has broken down) and the haemoglobin has passed into the kidney tubules.   • The presence of white blood cells in the urine indicates there is an infection in the kidney or urinary tract.   • Protein molecules are usually too large to pass into the kidney tubule so no or only small amounts of proteins  like albumin is normally found in urine. Large quantities of albumin indicate that the kidney tubules have  been injured or the kidney has become diseased. High blood pressure also pushes proteins from the blood into   • Casts are tiny cylinders of material that have been shed from the lining of the tubules and flushed out into the  urine.   • Mucus is not usually found in the urine of healthy animals but is a normal constituent of horses' urine, giving  it a characteristic cloudy appearance.   Tests can be carried out to identify any abnormal ingredients of urine. These tests are normally done by "stix", which  are small plastic strips with absorbent ends impregnated with various chemicals. A colour change occurs in the  presence of an abnormal ingredient.   Birds' high body temperature and level of activity means that they need to conserve water. Birds therefore do not  have a bladder and instead of excreting urea, which needs to be dissolved in large amounts of water, birds produce  uric acid that can be discharged as a thick paste along with the feces. This is the white chalky part of the bird  droppings that land on you or your car.   • The excretory system consists of paired kidneys and associated blood supply. Ureters transport urine from the  kidneys to the bladder and the urethra with associated sphincter muscles controls the release of urine.   • The kidneys have an important role in maintaining homeostasis in the body. They excrete the waste product urea,  control the concentrations of water and salt in the body fluids, and regulate the acidity of the blood.   • A kidney consists of an outer region or cortex, inner medulla and a cavity called the pelvis that collects the urine  and carries it to the ureter.   • The tissue of a kidney is composed of masses of tiny tubes called kidney tubules or nephrons. These are the  structures that make the urine.   • High-pressure blood is supplied to the nephron via a tuft of capillaries called the glomerulus. Most of the  contents of the blood except the cells and large protein molecules filter from the glomerulus into the (Bowmans)  capsule. This fluid flows down a coiled part of the tubule (proximal convoluted tubule) where useful substances  like glucose, amino acids and various ions are reabsorbed. The fluid flows to a looped portion of the tubule called  the Loop of Henle where water is reabsorbed and then to another coiled part of the tubule (distal convoluted  tubule) where more reabsorbtion and secretion takes place. Finally the fluid passes down the collecting duct  where water is reabsorbed to form concentrated urine.   Use this Excretory System Worksheet to help you learn the parts of the urinary system, the kidney and kidney  tubule and their functions.   The Urinary System Test Yourself can then be used to see if you understand this rather complex system.   1. Add the following labels to the diagram of the excretory system shown below. Bladder I ureter I urethra I kidney I  dorsal aorta I vena cava I renal artery I vein    2. Using the words/phrases in the list below fill in the blanks in the following statements.  I cortex I amino acids I renal I glucose I water reabsorption I large proteins I  I bowman's capsule I diabetes mellitus I secreted I antidiuretic hormone (ADH) I blood cells I  I glomerulus I concentration of the urine I medulla I nephron I   b) When cut across the kidney is seen to consist of two regions, the outer and the inner   f) The filtrate entering the e) above is similar to blood but does not contain or   g) As the fluid passes along the first coiled part of the kidney tubule and are removed.   i) Hydrogen and potassium ions are into the second coiled part of the tubule.   k) The hormone is responsible for controlling water reabsorption in the collecting tube.   1) When the pancreas secretes inadequate amounts of the hormone insulin the condition known as   results. This is most easily diagnosed by testing for in the urine.   • http://www.biologycorner.com/bio3/nephron.html Biology Corner. A fabulous drawing of the kidney and  nephron to print off, label and colour in with clear explanation of function.   • http://health.howstuffworks.com/adam-200032.htm How Stuff Works. This animation traces the full process  of urine formation and reabsorption in the kidneys, its path down the ureter to the bladder, and its excretion via  the urethra. Needs Shockwave.   • http://en.wikipedia.org/wiki/Nephron Wikipedia. A bit more detail than you need but still good clear  explanations and lots of information.   1. Add the following labels to the diagram of the excretory system shown below. Bladder I ureter I urethra I kidney I  dorsal aorta I vena cava I renal artery I vein   2. Fill in the blanks in the following statements,  a) Blood enters the kidney via the renal artery.   b) When cut across the kidney is seen to consist of two regions, the outer cortex and the inner medulla.   f) The filtrate entering the e) above is similar to blood but does not contain large proteins or blood cells.   g) As the fluid passes along the first coiled part of the kidney tubule amino acids and glucose are removed,  h) The main function of the loop of Henle is water re-absorption/concentration of urine.   i) Hydrogen and potassium ions are secreted into the second coiled part of the tubule.   j) The main function of the collecting tube is concentration of the urine.   k) The hormone antidiuretic hormone (ADH) is responsible for controlling the concentration of urine in the  collecting tube.   1) When the pancreas secretes inadequate amounts of the hormone insulin the condition known as diabetes mellitus  results. This is most easily diagnosed by testing for glucose in the urine.   • the role of mitosis and meiosis in the production  of gametes (sperm and ova)  that gametes are haploid cells  that fertilization forms a diploid zygote  the major parts of the male reproductive system  and their functions   the route sperm travel along the male reproductive  tract to reach the penis  the structure of a sperm and the difference  between sperm and semen  the difference between infertility and impotence  the main parts of the female reproductive system  and their functions   the ovarian cycle and the roles of FSH, LH,  oestrogen and progesterone  the oestrous cycle and the signs of heat in rodents,  dogs, cats and cattle   the process of fertilization and where it occurs in  the female tract   what a morula and a blastocyst are  what the placenta is and its functions   In biological terms sexual reproduction involves the union of gametes - the sperm and the ovum - produced by two  parents. Each gamete is formed by meiosis (see Chapter 3). This means each contains only half the chromosomes of  the body cells (haploid). Fertilization results in the joining of the male and female gametes to form a zygote which  contains the full number of chromosomes (diploid). The zygote then starts to divide by mitosis (see Chapter 3) to  form a new animal with all its body cells containing chromosomes that are identical to those of the original zygote  (see diagram 13.1).   The offspring formed by sexual reproduction contain genes from both parents and show considerable variation. For  example, kittens in a litter are all different although they (usually) have the same mother and father. In the wild this  variation is important because it means that when the environment changes some individuals may be better adapted  to survive than others. These survivors pass their "superior" genes on to their offspring. In this way the characteristics  of a group of animals can gradually change over time to keep pace with the changing environment. This "survival of  the fittest" or "natural selection" is the mechanism behind the theory of evolution.   In most fish and amphibia (frogs and toads) fertilisation of the egg cells takes place outside the body. The female  lays the eggs and then the male deposits his sperm on or at least near them.   In reptiles and birds, eggs are fertilized inside the body when the male deposits the sperm inside the egg duct of the  female. The egg is then surrounded by a resistant shell, "laid" by the female and the embryo completes its  development inside the egg.   In mammals the sperm are placed in the body of the female and the eggs are fertilized internally. They then develop  to quite an advanced stage inside the body of the female. When they are born they are fed on milk excreted from the  mammary glands and protected by their parents until they become independent.   The reproductive organs of mammals produce the gametes (sperm and egg cells), help them fertilise and then  support the developing embryo.   The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that transport  the sperm to the penis and glands that add secretions to the sperm to make semen (see diagram 13.2).   The various parts of the male reproductive system with a summary of their functions are shown in diagram 13.3.   Diagram 13.3 - Diagram summarising the functions of the male reproductive organs   Sperm need temperatures between 2 to 10 degrees Centigrade lower and then the body temperature to develop. This  is the reason why the testes are located in a bag of skin called the scrotal sacs (or scrotum) that hangs below the  body and where the evaporation of secretions from special glands can further reduce the temperature. In many  animals (including humans) the testes descend into the scrotal sacs at birth but in some animals they do not descend  until sexual maturity and in others they only descend temporarily during the breeding season. A mature animal in  which one or both testes have not descended is called a cryptorchid and is usually infertile.   The problem of keeping sperm at a low enough temperature is even greater in birds that have a higher body  temperature than mammals. For this reason bird's sperm are usually produced at night when the body temperature is  lower and the sperm themselves are more resistant to heat.   The testes consist of a mass of coiled tubes (the seminiferous or sperm producing tubules) in which the sperm are  formed by meiosis (see diagram 13.4). Cells lying between the seminiferous tubules produce the male sex hormone  testosterone.   When the sperm are mature they accumulate in the collecting ducts and then pass to the epididymisbefore moving  to the sperm duct or vas deferens. The two sperm ducts join the urethra just below the bladder, which passes  through the penis and transports both sperm and urine.   Ejaculation discharges the semen from the erect penis. It is brought about by the contraction of the epididymis, vas  deferens, prostate gland and urethra.   Semen consists of 10% sperm and 90% fluid and as sperm pass down the ducts from testis to penis, (accessory)  glands add various secretion...   Three different glands may be involved in producing the secretions in which sperm are suspended, although the  number and type of glands varies from species to species.   Seminal vesicles are important in rats, bulls, boars and stallions but are absent in cats and dogs. When present they  produce secretions that make up much of the volume of the semen, and transport and provide nutrients for the sperm.   The prostate gland is important in dogs and humans. It produces an alkaline secretion that neutralizes the acidity of  the male urethra and female vagina.   Cowper's glands (bulbourethral glands) have various functions in different species. The secretions may lubricate,  flush out urine or form a gelatinous plug that traps the semen in the female reproductive system after copulation and  prevents other males of the same species fertilizing an already mated female. Cowper's glands are absent in bears  and aquatic mammals.   The penis consists of connective tissue with numerous small blood spaces in it. These fill with blood during sexual  excitement causing erection.   Dogs, bears, seals, bats and rodents have a special bone in the penis which helps maintain the erection (see diagram  13.2). In some animals (e.g. the bull, ram and boar) the penis has an "S" shaped bend that allows it to fold up when  not in use. In many animals the shape of the penis is adapted to match that of the vagina. For example, the boar has a  corkscrew shaped penis, there is a pronounced twist in bulls' and it is forked in marsupials like the opossum. Some  have spines, warts or hooks on them to help keep them in the vagina and copulation may be extended to help retain  the semen in the female system. Mating can last up to three hours in minks, and dogs may "knot" or "tie" during  mating and can not separate until the erection has subsided.   Sperm are made up of three parts: a head consisting mainly of the nucleus, a midpiece containing many  mitochondria to provide the energy and a tail that provides propulsion (see diagram 13.5).   A single ejaculation may contain 2-3 hundred million sperm but even in normal semen as many as 10% of these  sperm may be abnormal and infertile. Some may be dead while others are inactive or deformed with double, giant or  small heads or tails that are coiled or absent altogether.   When there are too many abnormal sperm or when the sperm concentration is low, the semen may not be able to  fertilize an egg and the animal is infertile. Make sure you don't confuse infertility with impotence, which is the  inability to copulate successfully.   Sperm do not live forever. They have a definite life span that varies from species to species. They survive for  between 20 days (guinea pig) to 60 days (bull) in the epididymis but once ejaculated into the female tract they only  live from 12 to 48 hours. When semen is used for artificial insemination, storage under the right conditions can  extend the life span of some species.   In many species the male can be artificially stimulated to ejaculate and the semen collected. It can then be diluted,  stored and used to inseminate females. For example bull semen can be diluted and stored for up to 3 weeks at room  temperature. If mixed with an antifreeze solution and stored in "straws" in liquid nitrogen at minus 79 C it will keep  for much longer. Unfortunately the semen of chickens, stallions and boars can only be stored for up to 2 days.   Dilution of the semen means that one male can be used to fertilise many more females than would occur under  natural conditions. There are also advantages in the male and female not having to make physical contact. It means  that owners of females do not have to buy expensive males and the possibility of transmitting sexually transmitted  diseases is reduced. Routine examination of the semen for sperm concentration, quality and activity allows only the  highest quality semen to be used so a high success rate is ensured.   Since the lifespan of sperm in the female tract is so short and ova only survive from 8 to 10 hours the timing of the  artificial insemination is critical. Successful conception depends upon detecting the time that the animal is "on heat"  and when ovulation occurs.   The female reproductive system consists of a pair of ovaries that produce egg cells or ova and fallopian tubes  where fertilisation occurs and which carry the fertilised ovum to the uterus. Growth of the foetus takes place here.  The cervix separates the uterus from the vagina or birth canal, where the sperm are deposited (see diagram 13.6).   Note that primates like humans have a uterus with a single compartment but in most mammals the uterus is divided  into two separate parts or horns as shown in diagram 13.6.   Ovaries are small oval organs situated in the abdominal cavity just ventral to the kidneys. Most animals have a pair  of ovaries but in birds only the left one is functional to reduce weight (see below).   The ovary consists of an inner region (medulla) and an outer region (cortex) containing egg cells or ova. These are  formed in large numbers around the time of birth and start to develop after the animal becomes sexually mature. A  cluster of cells called the follicle surrounds and nourishes each ovum.   The ovarian cycle refers to the series of changes in the ovary during which the follicle matures, the ovum is shed  and the corpus luteum develops (see diagram 13.7).   Numerous undeveloped ovarian follicles are present at birth but they start to mature after sexual maturity. In animals  that normally have only one baby at a time only one ovum will mature at once but in litter animals several will. The  mature follicle consists of outer cells that provide nourishment. Inside this is a fluid-filled space that contains the  ovum.   A mature follicle can be quite large, ranging from a few millimetres in small mammals to the size of a golf ball in  large animals. It bulges out from the surface of the ovary before eventually rupturing to release the ovum into the  abdominal cavity. Once the ovum has been shed, a blood clot forms in the empty follicle. This develops into a tissue   called the corpus luteum that produces the hormone progesterone (see diagram 13.9). If the animal becomes  pregnant the corpus luteum persists, but if there is no pregnancy it degenerates and a new ovarian cycle usually.   Diagram 13.7 - The ovarian cycle showing from the top left clockwise: the maturation of the ovum over time,  followed by ovulation and the development of the corpus luteum in the empty follicle   When the ovum is shed the nucleus is in the final stages of meiosis (cell division). It is surrounded by few layers of  follicle cells and a tough membrane called the zona pellucida (see diagram 13.8).   The oestrous cycle is the sequence of hormonal changes that occurs through the ovarian cycle. These changes  influence the behaviour and body changes of the female (see diagram 13.9).   The first hormone involved in the oestrous cycle is follicle stimulating hormone (F.S.H.), secreted by the anterior   pituitary gland (see chapter 16). It stimulates the follicle to develop. As the follicle matures the outer cells begin to  secrete the hormone oestrogen and this stimulates the mammary glands to develop. It also prepares the lining of the  uterus to receive a fertilised egg. Ovulation is initiated by a surge of another hormone from the anterior pituitary,  luteinising hormone (L.H.)- This hormone also influences the development of the corpus luteum, which produces  progesterone, a hormone that prepares the lining of the uterus for the fertilised ovum and readies the mammary  glands for milk production. If no pregnancy takes place the corpus luteum shrinks and the production of  progesterone decreases. This causes FSH to be produced again and a new oestrous cycle begins.   For fertilisation of the ovum by the sperm to occur, the female must be receptive to the male at around the time of  ovulation. This is when the hormones turn on the signs of "heat", and she is "in season" or "in oestrous". These signs  are turned off again at the end of the oestrous cycle.   During the oestrous cycle the lining of the uterus (endometrium) thickens ready for the fertilised ovum to be  implanted. If no pregnancy occurs this thickened tissue is absorbed and the next cycle starts. In humans and other  higher primates, however, the endometrium is shed as a flow of blood and instead of an oestrous cycle there is a  menstrual cycle.   The length of the oestrous cycle varies from species to species. In rats the cycle only lasts 4-5 days and they are  sexually receptive for about 14 hours. Dogs have a cycle that lasts 60-70 days and heat lasts 7-9 days and horses  have a 21 -day cycle and heat lasts an average of 6 days.   Ovulation is spontaneous in most animals but in some, e.g. the cat, and the rabbit, ovulation is stimulated by mating.  This is called induced ovulation.   • When on heat a bitch has a blood stained discharge from the vulva that changes a little later to a straw  coloured one that attracts all the dogs in the neighbourhood.   • Female cats "call" at night, roll and tread the carpet and are generally restless but will "stand" firm when  pressure is placed on the pelvic region (this is the lordosis response).   • A female rat shows the lordosis response when on heat. It will "mount" other females and be more active than  normal.   • A cow mounts other cows (bulling), bellows, is restless and has a discharge from the vulva.   Only a few animals breed throughout the year. This includes the higher primates (humans, gorillas and chimpanzees  etc.), pigs, mice and rabbits. These are known as continuous breeders.   Most other animals restrict reproduction to one or two seasons in the year-seasonal breeders (see diagram 13.10).  There are several reasons for this. It means the young can be born at the time (usually spring) when feed is most  abundant and temperatures are favourable. It is also sensible to restrict the breeding season because courtship,  mating, gestation and the rearing of young can exhaust the energy resources of an animal as well as make them more  vulnerable to predators.   The timing of the breeding cycle is often determined by day length. For example the shortening day length in autumn  will bring sheep and cows into season so the foetus can gestate through the winter and be born in spring. In cats the  increasing day length after the winter solstice (shortest day) stimulates breeding. The number of times an animal  comes into season during the year varies, as does the number of oestrous cycles during each season. For example a  dog usually has 2-3 seasons per year, each usually consisting of just one oestrous cycle. In contrast ewes usually  restrict breeding to one season and can continue to cycle as many as 20 times if they fail to become pregnant.   The opening of the fallopian tube lies close to the ovary and after ovulation the ovum is swept into its funnel-like  opening and is moved along it by the action of cilia and wave-like contractions of the wall.   Copulation deposits several hundred million sperm in the vagina. They swim through the cervix and uterus to the  fallopian tubes moved along by whip-like movements of their tails and contractions of the uterus. During this  journey the sperm undergo their final phase of maturation so they are ready to fertilise the ovum by the time they  reach it in the upper fallopian tube.   High mortality means only a small proportion of those deposited actually reach the ovum. The sperm attach to the  outer zona pellucida and enzymes secreted from a gland in the head of the sperm dissolve this membrane so it can  enter. Once one sperm has entered, changes in the zona pellucida prevent further sperm from penetrating. The  sperm loses its tail and the two nuclei fuse to form a zygote with the full set of paired chromosomes restored.   As the fertilised egg travels down the fallopian tube it starts to divide by mitosis. First two cells are formed and then  four, eight, sixteen, etc. until there is a solid ball of cells. This is called a morula. As division continues a hollow  ball of cells develops. This is a blastocyst (see diagram 13.11).   Implantation involves the blastocyst attaching to, and in some species, completely sinking into the wall of the uterus.   As the embryo increases in size, the placenta, umbilical cord and foetal membranes (often known collectively as  the placenta) develop to provide it with nutrients and remove waste products (see diagram 13.12). In later stages of  development the embryo becomes known as a foetus.   The placenta is the organ that attaches the foetus to the wall of the uterus. In it the blood of the foetus and mother  flow close to each other but never mix (see diagram 13.13). The closeness of the maternal and foetal blood systems  allows diffusion between them. Oxygen and nutrients diffuse from the mother's blood into that of the foetus and  carbon dioxide and excretory products diffuse in the other direction. Most maternal hormones (except adrenaline),  antibodies, almost all drugs (including alcohol), lead and DDT also pass across the placenta. However, it protects the  foetus from infection with bacteria and most viruses.   The foetus is attached to the placenta by the umbilical cord. It contains arteries that carry blood to the placenta and a  vein that returns blood to the foetus. The developing foetus becomes surrounded by membranes. These enclose the  amniotic fluid that protects the foetus from knocks and other trauma (see diagram 13.12).   Diagram 13.13 - Maternal and foetal blood flow in the placenta   The corpus luteum continues to secrete progesterone and oestrogen during pregnancy. These maintain the lining of  the uterus and prepare the mammary glands for milk secretion. Later in the pregnancy the placenta itself takes over  the secretion of these hormones.   Chorionic gonadotrophin is another hormone secreted by the placenta and placental membranes. It prevents uterine  contractions before labour and prepares the mammary glands for lactation. Towards the end of pregnancy the  placenta and ovaries secrete relaxin, a hormone that eases the joint between the two parts of the pelvis and helps  dilate the cervix ready for birth.   The easiest method of pregnancy detection is ultrasound which is noninvasive and very reliable Later in gestation  pregnancy can be detected by taking x-rays.   In dogs and cats a blood test can be used to detect the hormone relaxin.   In mares and cows palpation of the uterus via the rectum is the classic way to determine pregnancy. It can also be  done by detecting the hormones progesterone or equine chorionic gonagotrophin (eCG) in the urine. A new  sensitive test measures the amount of the hormone, oestrone sulphate, present in a sample of faeces. The hormone is  produced by the foal and placenta, and is only present when there is a living foal.   In most animals, once pregnancy is advanced, there is a window of time during which an experienced veterinarian  can determine pregnancy by feeling the abdomen.   The young of many animals (e.g. pigs, horses and elephants) are born at an advanced state of development, able to  stand and even run to escape predators soon after they are born. These animals have a relatively long gestation  period that varies with their size e.g. from 1 14 days in the pig to 640 days in the elephant.   In contrast, cats, dogs, mice, rabbits and higher primates are relatively immature when born and totally dependent on  their parents for survival. Their gestation period is shorter and varies from 25 days in the mouse to 3 1 days in rabbits  and 258 days in the gorilla.   The babies of marsupials are born at an extremely immature stage and migrate to the pouch where they attach to a  teat to complete their development. Kangaroo joeys, for example, are born 33 days after conception and opossums  after only 8 days.   As the pregnancy continues, the mammary glands enlarge and may secrete a milky substance a few days before birth  occurs. The vulva may swell and produce thick mucus and there is sometimes a visible change in the position of the  foetus. Just before birth the mother often becomes restless, lying down and getting up frequently. Many animals seek  a secluded place where they may build a nest in which to give birth.   Labour involves waves of uterine contractions that press the foetus against the cervix causing it to dilate. The foetus  is then pushed through the cervix and along the vagina before being delivered. In the final stage of labour the  placenta or "afterbirth" is expelled.   The foetus grows in the watery, protected environment of the uterus where the mother supplies oxygen and nutrients,  and waste products pass to her blood circulation for excretion. Once the baby animal is born it must start to breathe  for itself, digest food and excrete its own waste. To allow these functions to occur blood is re-routed to the lungs and  the glands associated with the gut start to secrete. Note that newborn animals can not control their own body  temperature. They need to be kept warm by the mother, littermates and insulating nest materials.   Cows, manatees and primates have two mammary glands but animals like pigs that give birth to large litters may  have as many as 12 pairs. Ducts from the gland lead to a nipple or teat and there may be a sinus where the milk  collects before being suckled (see diagram 13.14).   The hormones oestrogen and progesterone stimulate the mammary glands to develop and prolactin promotes the  secretion of the milk. Oxytocin from the pituitary gland releases the milk when the baby suckles. The first milk is  called colostrum. It is a rich in nutrients and contains protective antibodies from the mother. Milk contains fat,  protein and milk sugar as well as vitamins and most minerals although it contains little iron. Its actual composition  varies widely from species to species. For example whale's and seal's milk has twelve times more fat and four times  more protein than cow's milk. Cow's milk has far less protein in it than cat's or dog's milk. This is why orphan  kittens and puppies cannot be fed cow's milk.   Male birds have testes and sperm ducts and male swans, ducks, geese and ostriches have a penis. However, most  birds make do with a small amount of erectile tissue known as a papilla. To reduce weight for flight most female  birds only have one ovary - usually the left, which produces extremely yolky eggs. The eggs are fertilised in the  upper part of the oviduct (equivalent to the fallopian tube and uterus of mammals) and as they pass down it albumin  (the white of the egg), the membrane beneath the shell and the shell are laid down over the yolk. Finally the egg is  covered in a layer of mucus to help the bird lay it (see diagram 13.15).   Most birds lay their eggs in a nest and the hen sits on them until they hatch. Ducklings and chicks are relatively well  developed when they hatch and able to forage for their own food. Most other nestlings need their parents to keep  them warm, clean and fed. Young birds grow rapidly and have voracious appetites that may involve the parents  making up to 1000 trips a day to supply their need for food.   • Haploid gametes (sperm and ova) are produced by meiosis in the gonads (testes and ovaries).   • Fertilisation involves the fusing of the gametes to form a diploid zygote.   • The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that  transport the sperm to the penis and glands that add secretions to the sperm to make semen.   • Sperm are produced in the seminiferous tubules, are stored in the epididymis and travel via the vas deferens or  sperm duct to the junction of the bladder and the urethra where various accessory glands add secretions. The  fluid is now called semen and is ejaculated into the female system down the urethra that runs down the centre of  the penis.   • Sperm consist of a head, a midpiece and a tail.   • Infertility is the inability of sperm to fertilize an egg while impotence is the inability to copulate successfully.   • The female reproductive system consists of a pair of ovaries that produce ova and fallopian tubes where  fertilisation occurs and which carry the fertilised ovum to the uterus. Growth of the foetus takes place here. The  cervix separates the uterus from the vagina, the birth canal and where the sperm are deposited.   • The ovarian cycle refers to the series of changes in the ovary during which the follicle matures, the ovum is shed  and the corpus luteum develops.   • The oestrous cycle is the sequence of hormonal changes that occurs through the ovarian cycle. It is initiated by  the secretion of follicle stimulating hormone (F.S.H.), by the anterior pituitary gland which stimulates the  follicle to develop. The follicle secretes oestrogen which stimulates mammary gland development, luteinising  hormone (L.H.) from the anterior pituitary initiates ovulation and stimulates the corpus luteum to develop. The  corpus luteum produces progesterone that prepares the lining of the uterus for the fertilised ovum.   • Signs of oestrous or heat differ. A bitch has a blood stained discharge, female cats and rats are restless and show  the lordosis response, while cows mount other cows, bellow and have a discharge from the vulva.   • After fertilisation in the fallopian tube the zygote divides over and over by mitosis to become a ball of cells called  a morula. Division continues to form a hollow ball of cells called the blastocyst. This is the stage that implants  in the uterus.   • The placenta, umbilical cord and foetal membranes (known as the placenta) protect and provide the  developing foetus with nutrients and remove waste products.   1. Add the following labels to the diagram of the male reproductive organs below.   testis I epididymis I vas deferens I urethra I penis I scrotal sac I prostate gland  Diagram of the Male Reproductive System   2. Match the following descriptions with the choices given in the list below.   accessory glands I vas deferens or sperm duct I penis I scrotum I fallopian tube I testes I urethra I vagina I  uterus I ovary I vulva   e) Glands that produce secretions that make up most of the semen   a) This hormone stimulates the growth of the follicles in the ovary   b) This hormone converts the empty follicle into the corpus luteum and stimulates it to produce progesterone   c) This hormone is produced by the cells of the follicle   f) This hormone prepares the lining of the uterus to receive a fertilised ovum   4. State whether the following statements are true or false. If false write in the correct answer.   b) The fertilised egg cell contains half the normal number of chromosomes   d) The mixing of the blood of the mother and foetus allows nutrients and oxygen to transfer easily to the foetus   f) The placenta is the organ that supplies the foetus with oxygen and nutrients   h) Young animals often have to be given calcium supplements because milk contains very little calcium  Test Yourself Answers   • http://www.anatomicaltravel.com/CB_site/Conception_to_birth3.htm Anatomical travel. Images of  fertilisation and the development of the (human) embryo through to birth.   • http://www.uchsc.edu/ltc/fert.swf Fertilisation. A great animation of fertilisation, formation of the zygote and  first mitotic division. A bit advanced but still worth watching.   • http://www.uclan.ac.uk/facs/health/nursing/sonic/scenarios/salfordanim/heart.swf Sonic. An animation  showing the foetal blood circulation through the placenta to the changes allowing circulation through the lungs  after birth.   • http://en.wikipedia.org/wiki/Estrus Wikipedia. As always, good interesting information although some terms  and concepts are beyond the requirements of this level.   1. Add the following labels to the diagram of the male reproductive organs below.   testis I epididymis I vas deferens I urethra I penis I scrotal sac I prostate gland  Diagram of Male Reproductive System with labels added   2. Match the following descriptions with the choices given in the list below.   accessory glands I vas deferens or sperm duct I penis I scrotum I fallopian tube I testes I urethra I vagina I uterus  I ovary I vulva   c) The vas deferens or sperm duct is the passage for sperm from the epididymis to the penis.   d) The urethra carries both sperm and urine down the penis.   e) Accessory glands produce secretions that make up most of the semen.   f) The scrotum is the bag of skin surrounding the testes.   j) Ova travel along the fallopian tube to reach the uterus.   a) Follicle stimulating hormone (FSH) stimulates the growth of the follicles in the ovary.   b) Luteinising hormone (LH) converts the empty follicle into the corpus luteum and stimulates it to produce  progesterone.   f) Progesterone prepares the lining of the uterus to receive a fertilised ovum.   4. State whether the following statements are true or false. If false write in the correct answer.   b) The fertilised egg cell contains half the normal number of chromosomes.   False - Once fertilised the normal (diploid or 2n) number of chromosomes are restored.   False - The morula is a solid ball of cells. It is the blastocyst that is hollow.   d) Mixing of the blood of the mother and foetus allows nutrients and oxygen to transfer easily to the foetus.   False - There is no mixing of foetal and maternal blood in the placenta.   False. It is the blastocyst that implants in the wall of the uterus.   f) The placenta is the organ that supplies the foetus with oxygen and nutrients.   g) To adjust the baby animal to the outside world the blood has to be re-routed to the lungs.   True. In the uterus the foetal lungs are inactive and once the baby is born and starts to breathe blood has to be  supplied to the lungs so it can be oxygenated.   True.  i) Young animals often have to be given calcium supplements because milk contains very little calcium.   False. Milk contains lots of calcium. What it lacks is iron and this often has to be given as a supplement.   the basic parts of the brain and the function of the   that the autonomic nervous system consists of sympathetic and parasympathetic parts with different functions   Animals must be able to sense and respond to the environment in which they live if they are to survive. They need to  be able to sense the temperature of their surroundings, for example, so they can avoid the hot sun. They must also be  able to identify food and escape predators.   The various systems and organs in the body must also be linked so they work together. For example, once a predator  has identified suitable prey it has to catch it. This involves coordinating the contraction of the muscle so the predator  can run, there must then be an increased blood supply to the muscles to provide them with oxygen and nutrients. At  the same time the respiration rate must increase to supply the oxygen and remove the carbon dioxide produced as a  result of this increased activity. Once the prey has been caught and eaten, the digestive system must be activated to  digest it.   The adjustment of an animal's response to changes in the environment and the complex linking of the various  processes in the body that this response involves are called co-ordination. Two systems are involved in  co-ordination in animals. These are the nervous and endocrine systems. The first operates via electrical impulses  along nerve fibres and the second by releasing special chemicals or hormones into the bloodstream from glands.   1 . Sensory function - to sense changes (known as stimuli) both outside and within the body. For example the  eyes sense changes in light and the ear responds to sound waves. Inside the body, tretch receptors in the  stomach indicate when it is full and chemical receptors in the blood vessels monitor the acidity of the blood.   2. Integrative function - processing the information received from the sense organs. The impulses from these  organs are analysed and stored as memory. The many different impulses from different sources are sorted,  synchronised and co-ordinated and the appropriate response initiated. The power to integrate, remember and  apply experience gives higher animals much of their superiority.   3. Motor function - The third function is the response to the stimuli that causes muscles to contract or glands  to secrete.   All nervous tissue is made up of nerve cells or neurons. These transmit high-speed signals called nerve impulses.  Nerve impulses can be thought of as being similar to an electric current.   Neurons are cells that have been adapted to carry nerve impulses. A typical neuron has a cell body containing a  nucleus, one or more branching filaments called dendrites which conduct nerve impulses towards the cell body and  one long fibre, an axon, that carries the impulses away from it. Many axons have a sheath of fatty material called  myelin surrounding them. This speeds up the rate at which the nerve impulses travel along the nerve (see diagram  14.1).   The cell body of neurons is usually located in the brain or spinal cord while the axon extends the whole distance to  the organ that it supplies. The neuron carrying impulses from the spinal cord to the hind leg or tail of a horse, for  example, can be several feet long. A nerve is a bundle of axons.   A sensory neuron is a nerve cell that transmits impulses from a sense receptor such as those in the eye or ear to the  brain or spinal cord. A motor neuron is a nerve cell that transmits impulses from the brain or spinal cord to a muscle  or gland. A relay neuron connects sensory and motor neurons and is found in the brain or spinal cord (see diagrams  14.1 and 14.2).   Diagram 14.2 - The relationship between sensory, relay and motor neurons   The connection between adjacent neurons is called a synapse. The two nerve cells do not actually touch here for  there is a microscopic space between them. The electrical impulse in the neurone before the synapse stimulates the  production of chemicals called neurotransmitters (such as acetylcholine), which are secreted into the gap.   The neurotransmitter chemicals diffuse across the gap and when they contact the membrane of the next nerve cell  they stimulate a new nervous impulse (see diagram 14.3). After the impulse has passed the chemical is destroyed and  the synapse is ready to receive the next nerve impulse.   A reflex is a rapid automatic response to a stimulus. When you accidentally touch a hot object and automatically jerk  your hand away, this is a reflex action. It happens without you having to think about it. Animals automatically blink  when an object approaches the eye and cats twist their bodies in the air when falling so they land on their paws.  (Please don't test this one at home with your pet cat!).   Swallowing, sneezing, and the constriction of the pupil of the eye in bright light are also all reflex actions.   The path taken by the nerve impulses in a reflex is called a reflex arc. Most reflex arcs involve only three neurons  (see diagram 14.4). The stimulus (a pin in the paw) stimulates the pain receptors of the skin, which initiate an  impulse in a sensory neuron. This travels to the spinal cord where it passes, by means of a synapse, to a connecting  neuron called the relay neuron situated in the spinal cord. The relay neuron in turn makes a synapse with one or more  motor neurons that transmit the impulse to the muscles of the limb causing them to contract and remove the paw  from the sharp object. Reflexes do not require involvement of the brain although you are aware of what is happening  and can, in some instances, prevent them happening. Animals are born with their reflexes. You can think of them as  being wired in.   In most reflexes the stimulus and response are related. For example the presence of food in the mouth causes the  salivary glands to release saliva. However, it is possible to train animals (and humans) to respond to different and  often quite irrelevant stimuli. This is called a conditioned reflex.   A Russian biologist called Pavlov carried out the classic experiment to demonstrate such a reflex when he  conditioned dogs to salivate at the sound of a bell ringing. Almost every pet owner can identify reflexes they have  conditioned in their animals. Perhaps you have trained your cat to associate food with the opening of the fridge door  or accustomed your dog to the routines you go through before taking them for a walk.   When we describe the nervous system of vertebrates we usually divide it into two parts (see diagram 14.5).   1 . The central nervous system (CNS) which consists of the brain and spinal cord.   2. The peripheral nervous system (PNS) which consists of the nerves that connect to the brain and spinal  cord (cranial and spinal nerves) as well as the autonomic (or involuntary) nervous system.   The central nervous system consists of the brain and spinal cord. It acts as a kind of 'telephone exchange' where a  vast number of cross connections are made.   When you look at the brain or spinal cord some regions appear creamy white (white matter) and others appear grey  (grey matter). White matter consists of masses of nerve axons and the grey matter consists of the nerve cells. In the  brain the grey matter is on the outside and in the spinal cord it is on the inside (see diagram 14.2).   The major part of the brain lies protected within the sturdy "box" of skull called the cranium. Surrounding the fragile  brain tissue (and spinal cord) are protective membranes called the meninges (see diagram 14.6), and a crystal-clear  fluid called cerebrospinal fluid, which protects and nourishes the brain tissue. This fluid also fills four cavities or  ventricles that lie within the brain.   Brain tissue is extremely active and, even when an animal is resting, it uses up to 20% of the oxygen taken into the  body by the lungs. The carotid artery, a branch off the dorsal aorta, supplies it with the oxygen and nutrients it  requires. Brain damage occurs if brain tissue is deprived of oxygen for only 4-8 minutes.   1 . the fore brain which includes the cerebral hemispheres, hypothalamus and pituitary gland;   2. the hind brain or brain stem, contains the medulla oblongata and pons and   In humans and some animals the functions of the different regions of the cerebral cortex have been mapped (see  diagram 14.7).   Diagram 14.7 - The functions of the regions of the human cerebral cortex   The cerebral hemispheres are the masses of brain tissue that sit on the top of the brain. The surface is folded into  ridges and furrows called sulci (singular sulcus). They make this part of the brain look rather like a very large walnut  kernel. The two hemispheres are separated by a deep groove although they are connected internally by a thick bundle  of nerve fibres. The outer layer of each hemisphere is called the cerebral cortex and this is where the main functions  of the cerebral hemispheres are carried out.   The cerebral cortex is large and convoluted in mammals compared to other vertebrates and largest of all in humans  because this is where the so-called "higher centres" concerned with memory, learning, reasoning and intelligence are  situated.   Nerves from the eyes, ears, nose and skin bring sensory impulses to the cortex where they are interpreted.  Appropriate voluntary movements are initiated here in the light of the memories of past events.   Different regions of the cortex are responsible for particular sensory and motor functions, e.g. vision, hearing, taste,  smell, or moving the fore-limbs, hind-limbs or tail. For example, when a dog sniffs a scent, sensory impulses from  the organ of smell in the nose pass via the olfactory (smelling) nerve to the olfactory centres of the cerebral  hemispheres where the impulses are interpreted and co-ordinated.   In humans and some animals the functions of the different regions of the cerebral cortex have been mapped (see  diagram 14.8).    Diagram 14.8 - The functions of the regions of the cerebral cortex   The hypothalamus is situated at the base of the brain and is connected by a "stalk" to the pituitary gland, the  "master" hormone-producing gland (see chapter 16). The hypothalamus can be thought of as the bridge between the  nervous and endocrine (hormone producing) systems. It produces some of the hormones that are released from the  pituitary gland and controls the release of others from it.   It is also an important centre for controlling the internal environment of the animal and therefore maintaining  homeostasis. For example, it helps regulate the movement of food through the gut and the temperature, blood  pressure and concentration of the blood. It is also responsible for the feeling of being hungry or thirsty and it controls  sleep patterns and sex drive.   The medulla oblongata is at the base of the brain and is a continuation of the spinal cord. It carries all signals  between the spinal cord and the brain and contains centres that control vital body functions like the basic rhythm of  breathing, the rate of the heartbeat and the activities of the gut. The medulla oblongata also co-ordinates swallowing,  vomiting, coughing and sneezing.   The cerebellum (little brain) looks rather like a smaller version of the cerebral hemispheres attached to the back of  the brain. It receives impulses from the organ of balance (vestibular organ) in the inner ear and from stretch receptors  in the muscles and tendons. By co-ordinating these it regulates muscle contraction during walking and running and  helps maintain the posture and balance of the animal. When the cerebellum malfunctions it causes a tremor and  uncoordinated movement.   The spinal cord is a cable of nerve tissue that passes down the channel in the vertebrae from the hindbrain to the end  of the tail. It becomes progressively smaller as paired spinal nerves pass out of the cord to parts of the body.  Protective membranes or meninges cover the cord and these enclose cerebral spinal fluid (see diagram 14.9).   If you cut across the spinal cord you can see that it consists of white matter on the outside and grey matter in the  shape of an H or butterfly on the inside.   The peripheral nervous system consists of nerves that are connected to the brain (cranial nerves), and nerves that  are connected to the spinal cord (spinal nerves). The autonomic nervous system is also part of the peripheral  nervous system.   There are twelve pairs of cranial nerves that come from the brain. Each passes through a hole in the cranium (brain  case). The most important of these are the olfactory, optic, acoustic and vagus nerves.   The olfactory nerves - (smell) carry impulses from the olfactory organ of the nose to the brain.   The optic nerves - (sight) carry impulses from the retina of the eye to the brain.   The auditory (acoustic) nerves - (hearing) carry impulses from the cochlear of the inner ear to the brain.   The vagus nerve - controls the muscles that bring about swallowing. It also controls the muscles of the heart,  airways, lungs, stomach and intestines (see diagram 14.5).   Spinal nerves connect the spinal cord to sense organs, muscles and glands in the body. Pairs of spinal nerves leave  the spinal cord and emerge between each pair of adjacent vertebrae (see diagram 14.9).   The sciatic nerve is the largest spinal nerve in the body (see diagram 14.5). It leaves the spinal cord as several  nerves that join to form a flat band of nervous tissue. It passes down the thigh towards the hind leg where it gives off  branches to the various muscles of this limb.   The autonomic nervous system controls internal body functions that are not under conscious control. For example  when a prey animal is chased by a predator the autonomic nervous system automatically increases the rate of  breathing and the heartbeat. It dilates the blood vessels that carry blood to the muscles, releases glucose from the  liver, and makes other adjustments to provide for the sudden increase in activity. When the animal has escaped and is  safe once again the nervous system slows down all these processes and resumes all the normal body activities like  the digestion of food.   The nerves of the autonomic nervous system originate in the spinal cord and pass out between the vertebrae to serve  the various organs (see diagram 14. 10). There are two main parts to the autonomic nervous system — the  sympathetic system and the parasympathetic system.   The sympathetic system stimulates the "flight, fright, fight" response that allows an animal to face up to an attacker  or make a rapid departure. It increases the heart and respiratory rates, as well as the amount of blood flowing to the  skeletal muscles while blood flow to less critical regions like the gut and skin is reduced. It also causes the pupils of  the eyes to dilate. Note that the effects of the sympathetic system are similar to the effects of the hormone adrenaline  (see Chapter 16).   The parasympathetic system does the opposite to the sympathetic system. It maintains the normal functions of the  relaxed body. These are sometimes known as the "housekeeping" functions. It promotes effective digestion,  stimulates defaecation and urination and maintains a regular heartbeat and rate of breathing.   Diagram 14.10 - The function of the sympathetic and parasympathetic nervous systems   • The neuron is the basic unit of the nervous system. It consists of a cell body with a nucleus, filaments known as  dendrites and a long fibre known as the axon often surrounded by a myelin sheath.   • Grey matter in the brain and spinal cord consists mainly of brain cells while white matter consists of masses of  axons.   • Reflexes are automatic responses to stimuli. The path taken by nerve impulses involved in reflexes is a reflex arc.  Most reflex arcs involve 3 neurons - a sensory neuron, a relay neuron and a motor neuron. A stimulus, a pin in  the paw for example, initiates an impulse in the sensory neuron that passes via a synapse to the relay neuron  situated in the spinal cord and then via another synapse to the motor neurone. This transmits the impulse to the  muscle causing it to contract and remove the paw from the pin.   • The nervous system is divided into 2 parts: the central nervous system, consisting of the brain and spinal cord  and the peripheral nervous system consisting of nerves connected to the brain and spinal cord. The autonomic  nervous system is considered to be part of the peripheral nervous system.   • The brain consists of three major regions: 1 . the fore brain which includes the cerebral hemispheres (or  cerebrum), hypothalamus and pituitary gland; 2. the hindbrain or brain stem containing the medulla  oblongata and 3. the cerebellum.   • Protective membranes known as the meninges surround the brain and spinal cord.   • There are 12 pairs of cranial nerves that include the optic, olfactory, acoustic and vagus nerves.   • The spinal cord is a cable of nerve tissue surrounded by meninges passing from the brain to the end of the tail.  Spinal nerves emerge by a ventral and dorsal root between each vertebra and connect the spinal cord with  organs and muscles.   • The autonomic nervous system controls internal body functions not under conscious control. It is divided into 2  parts with 2 different functions: the sympathetic nervous system that is involved in the flight and fight response  including increased heart rate, bronchial dilation, dilation of the pupil and decreased gut activity. The  parasympathetic nervous system is associated with decreased heart rate, pupil constriction and increased gut  activity.   1 . Add the following labels to this diagram of a motor neuron.   4. Rearrange the parts of a reflex arc given below in the order in which the nerve impulse travels from the sense  organ to the muscle.   5. Add the following labels to the diagram of the dog's brain shown below.   cerebellum I cerebral hemisphere I cerebral cortex I pituitary gland I medulla oblongata    6. What is the function of the meninges that cover the brain and spinal cord   7. Give 3 effects of the action of the sympathetic nervous system.  Test Yourself Answers   • http://en.wikipedia.org/wiki/Neuron Wikipedia. Lots of good information here but as usual a warning that   there are terms and concepts that are beyond the scope of this course. Also try 'reflex action' ; 'autonomic nervous  system' ;   images%3Fq%3Dneuron%26svnum%3D10%26hl%3Den%261r%3D%26sa%3DN How Stuff Works. This site is  for the neuron but try 'neuron types', 'brain parts' and 'balancing act' too.   • http://www.bbc.co.uk/schools/gcsebitesize/flash/bireflexarc.swf Reflex Arc. Nice clear and simple  animation of a reflex arc.   1 . Add the following labels to this diagram of a motor neuron.   4. Rearrange the parts of a reflex arc given below in the order in which the nerve impulse travels from the sense  organ to the muscle.   The nerve impulse travels from the sense organ to the sensory neuron to the relay neuron to the motor neuron  to the muscle fibres.   5. Add the following labels to the diagram of the dog's brain shown below.   cerebellum I cerebral hemisphere I cerebral cortex I pituitary gland I medulla oblongata   6. What is the function of the meninges that cover the brain and spinal cord   7. Give 3 effects of the action of the sympathetic nervous system.   The sympathetic nervous system increases the heart rate, the rate of breathing, the blood flow to skeletal  muscles and causes the pupils of the eye to dilate.   • that the general senses of touch, pressure, pain etc.  are situated in the dermis of the skin and in the  body   • that the special senses include those of smell,  taste, sight, hearing, and balance   • the route taken by light through the eye to the  retina   • the role of the rods and cones in the retina   • the route taken by sound waves through the ear to  the cochlea   • the role of the vestibular organ (semicircular  canals and otolith organ) in maintaining balance  and posture   Sense organs allow animals to sense changes in the  environment around them and in their bodies so that  they can respond appropriately. They enable animals  to avoid hostile environments, sense the presence of predators and find food.   Animals can sense a wide range of stimuli that includes, touch, pressure, pain, temperature, chemicals, light, sound,  movement and position of the body. Some animals can sense electric and magnetic fields. All sense organs respond  to stimuli by producing nerve impulses that travel to the brain via a sensory nerve. The impulses are then processed  and interpreted in the brain as pain, sight, sound, taste etc.   1. The general senses of touch, pressure, pain and temperature that are distributed fairly evenly through the  skin. Some are found in muscles and within joints.   2. The special senses which include the senses of smell, taste, sight, hearing and balance. The special sense  organs may be quite complex in structure.   Within the dermis of the skin are numerous modified nerve endings that are sensitive to touch and pressure. The  roots of hairs may also be well supplied with sensory receptors that inform the animal that it is in contact with an  object (see diagram 15.1). Whiskers are specially modified hairs.   Receptors that sense pain are found in almost every tissue of the body. They tell the animal that tissues are  dangerously hot, cold, compressed or stretched or that there is not enough blood flowing in them. The animal may  then be able to respond and protect itself from further damage   Nerve endings in the skin respond to hot and cold stimuli (See diagram 15.1).   There are sense organs in the muscles, tendons and joints that send continuous impulses to the brain that tell it where  each limb is. This information allows the animal to place its limbs accurately and know their exact position without  having to watch them.   Animals use the sense of smell to locate food, mark territory, identify their own offspring and the presence and  sexual condition of a potential mate. The organ of smell (olfactory organ) is located in the nose and responds to  chemicals in the air. It consists of modified nerve cells that have several tiny hairs on the surface. These emerge from  the epithelium on the roof of the nose cavity into the mucus that lines it. As the animal breathes, chemicals in the air  dissolve in the mucus. When the sense cell responds to a particular molecule, it fires an impulse that travels along the  olfactory nerve to the brain where it is interpreted as an odour (see diagram 15.2).   Diagram 15.2 - The olfactory organ - the sense of smell   The olfactory sense in humans is rudimentary compared to that of many animals. Carnivores that hunt have a very  highly developed sensitivity to scents. For example a polar bear can smell out a dead seal 20 km away and a  bloodhound can distinguish between the trails of different people although it may sometimes be confused by the  criss-crossing trail of identical twins.   Snakes and lizards detect odours by means of Jacobson's organ. This is situated on the roof of the mouth and  consists of pits containing sensory cells. When snakes flick out their forked tongues they are smelling the air by  carrying the molecules in it to the Jacobson's organ.   The sense of taste allows animals to detect and identify dissolved chemicals. In reptiles, birds, and mammals the  taste receptors (taste buds) are found mainly to the upper surface of the tongue. They consist of pits containing  sensory cells arranged rather like the segments of an orange (see diagram 15.3). Each receptor cell has a tiny "hair"  that projects into the saliva to sense the chemicals dissolved in it.   The sense of taste is quite restricted. Humans can only distinguish four different tastes (sweet, sour, bitter and salt)  and what we normally think of as taste is mainly the sense of smell. Food is quite tasteless when the nose is blocked  and cats often refuse to eat when this happens.   The eyes are the organs of sight. They consist of spherical eyeballs situated in deep depressions in the skull called  the orbits. They are attached to the wall of the orbit by six muscles, which move the eyeball. Upper and lower  eyelids cover the eyes during sleep and protect them from foreign objects or too much light, and spread the tears  over their surface.   The nictitating membrane or hawis a transparent sheet that moves sideways across the eye from the inner corner,  cleansing and moistening the cornea without shutting out the light. It is found in birds, crocodiles, frogs and fish as  well as marsupials like the kangaroo. It is rare in mammals but can be seen in cats and dogs by gently opening the  eye when it is asleep. Eyelashes also protect the eyes from the sun and foreign objects.   Lining the eyelids and covering the front of the eyeball is a thin epithelium called the conjunctiva. Conjunctivitis is  inflammation of this membrane. Tear glands that open just under the top eyelid secrete a salty solution that keeps  the exposed part of the eye moist, washes away dust and contains an enzyme that destroys bacteria.   The wall of the eyeball is composed of three layers (see diagram 15.4). From the outside these are the sclera, the  choroid and the retina.   The sclera is a tough fibrous layer that protects the eyeball and gives it rigidity. At the front of the eye the sclera is  visible as the "white" of the eye, which is modified as the transparent cornea through which the light rays have to  pass to enter the eye. The cornea helps focus light that enters the eye.   The choroid lies beneath the sclera. It contains a network of blood vessels that supply the eye with oxygen and  nutrients. Its inner surface is highly pigmented and absorbs stray light rays. In nocturnal animals like the cat and  possum this highly pigmented layer reflects light as a means of conserving light. This is what makes them shine  when caught in car headlights.   At the front of the eye the choroid becomes the iris. This is the coloured part of the eye that controls the amount of  light entering the pupil of the eye. In dim light the pupil is wide open so as much light as possible enters while in  bright light the pupils contract to protect the retina from damage by excess light.   The pupil in most animals is circular but in many nocturnal animals it is a slit that can close completely. This helps  protect the extra-sensitive light sensing tissues of animals like the cat and possum from bright sunlight.   The inner layer lining the inside of the eye is the retina. This contains the light sensing cells called rods and cones  (see diagram 15.5).   The rod cells are long and fat and are sensitive to dim light but cannot detect colour. They contain large amounts of  a pigment that changes when exposed to light. This pigment comes from vitamin A found in carrots etc. A deficiency  of this vitamin causes night blindness. So your mother was right when she told you to eat your carrots as they would  help you see in the dark!   The cone cells provide colour vision and allow animals to see details. Most are found in the centre of the retina and  they are most densely concentrated in a small area called the fovea. This is the area of sharpest vision, where the  words you are reading at this moment are focussed on your retina.   The nerve fibres from the cells of the retina join and leave the eye via the optic nerve. There are no rods or cones  here and it is a blind spot. The optic nerve passes through the back of the orbit and enters the brain.   The lens is situated just behind the pupil and the iris. It is a crystalline structure with no blood vessels and is held in  position by a ligament. This is attached to a muscle, which changes the shape of the lens so both near and distant  objects can be focussed by the eye. This ability to change the focus of the lens is called accommodation. In many  mammals the muscles that bring about accommodation are poorly developed, Rats, cows and dogs, for example, are  thought to be unable to focus clearly on near objects.   In old age and certain diseases the lens may become cloudy resulting in blurred vision. This is called a cataract.  Within the eyeball are two cavities, the anterior and posterior chambers, separated by the lens. They contain fluids  the aqueous and vitreous humoursrespectively, that maintain the shape of the eyeball and help press the retina   Eyes work quite like a camera. Light rays from an object enter the eye and are focused on the retina (the "film") at  the back of the eye. The cornea, the lens and the fluid within the eye all help to focus the light. They do this by  bending the light rays so that light from the object falls on the retina. This bending of light is called refraction. The  light stimulates the light sensitive cells of the retina and nerve impulses are produced that pass down the optic nerve  to the brain (see diagram 15.6).   Diagram 15.6 - How the light travels from the object to the retina of the eye   As mentioned before, the retina has two different kinds of cells that are stimulated by light - 'rods' and 'cones'. In  humans and higher primates like baboons and gorillas the rods function in dim light and do not perceive colour,  while the cones are stimulated by bright light and perceive details and colour.   Other mammals have very few cones in their retinas and it is believed that they see no or only a limited range of  colour. It is, of course, difficult to find out exactly what animals do see. It is thought that deer, rats, and rabbits and  nocturnal animals like the cat are colour-blind, and dogs probably see green and blue. Some fish and most birds seem  to have better colour vision than humans and they use colour, often very vivid ones, for recognizing each other as  well as for courtship and protection.   Animals like cats that hunt have eyes placed on the front of the head in such a way that both eyes see the same wide  area but from slightly different angles (see diagram 15.7). This is called binocular vision. Its main advantage is that  it enables the animals to estimate the distance to the prey so they can chase it and pounce accurately.   In contrast plant-eating prey animals like the rabbit and deer need to have a wide panoramic view so they can see  predators approaching. They therefore have eyes placed on the side of the head, each with its own field of vision (see  diagram 15.8). They have only a very small area of binocular vision in front of the head but are extremely sensitive  to movement.   Animals use the sense of hearing for many different purposes. It is used to sense danger and enemies, to detect prey,  to identify prospective mates and to communicate within social groups. Some animals (e.g. most bats and dolphins)  use sound to "see" by echolocation. By sending out a cry and interpreting the echo, they sense obstacles or potential  prey.   Most of the ear, the organ of hearing, is hidden from view within the boney skull. It consists of three main regions:  the outer ear, the middle ear and the inner ear (see diagram 15.9).   The outer ear consists of an ear canal leading inwards to a thin membrane known as the eardrumor tympanic   membrane that stretches across the canal. Many animals have an external ear flap or pinna to collect and funnel the  sound into the ear canal. The pinnae (plural of pinna) usually face forwards on the head but many animals can swivel  them towards the source of the sound.   In dogs the ear canal is long and bent and often traps wax or provides an ideal habitat for mites, yeast and bacteria.   The middle ear consists of a cavity in the skull that is connected to the pharynx (throat) by a long narrow tube  called the Eustachian tube. This links the middle ear to the outside air so that the air pressure on both sides of the  eardrum can be kept the same. Everyone knows the uncomfortable feeling (and affected hearing) that occurs when  you drive down a steep hill and the unequal air pressures on the two sides of the eardrum cause it to distort. The  discomfort is relieved when you swallow because the Eustachian tubes open and the pressure on either side  equalises.   Within the cavity of the middle ear are three of the smallest bones in the body, the auditory ossicles. They are  known as the hammer, the anvil and the stirrup because of their resemblance to the shape of these objects. These tiny  bones articulate (move against) each other and transfer the vibrations of the eardrum to the membrane covering the  opening to the inner ear.   The inner ear is a complicated series of fluid-filled tubes imbedded in the bone of the skull. It consists of two main  parts. These are the cochlea where sound waves are converted to nerve impulses and the vestibular organ that is   associated with the sense of balance and has no role in hearing (see later).   The cochlea looks rather like a coiled up snail shell. Within it there are specialised cells with fine hairs on their  surface that respond to the movement of the fluid within the cochlea by producing nervous impulses that travel to the  brain along the auditory nerve.   Sound waves can be thought of as vibrations in the air. They are collected by the ear pinna and pass down the ear  canal where they cause the eardrum to vibrate. (An interesting fact is that when you are listening to someone  speaking your eardrum vibrates at exactly the same rate as the vocal cords of the person speaking to you).   The vibration of the eardrum sets the three tiny bones in the middle ear moving against each other so that the  vibration is transferred to the membrane covering the opening to the inner ear. As well as transferring the vibration,  the tiny ear bones also amplify it. The three tiny bones are called the stirrup, anvil, and hammer. They were called  such of their form. In the human ear this amplification is about 20 times while in desert-dwelling animals like the  kangaroo rat it is 100 times. This acute hearing warns them of the approach of predators like owls and snakes, even  in the dark.   The vibration causes waves in the fluid in the inner ear that pass down the cochlea. These waves stimulate the tiny  hair cells to produce nerve impulses that travel via the auditory nerve to the cerebral cortex of the brain where they  are interpreted as sound.   To summarise: The route sound waves take as they pass through the ear is: External ear I tympanic membrane I  ear ossicles I inner ear Icochlear I hair cells   The hair cells generate a nerve impulse that travels down the auditory nerve to the brain.   Remember that sound waves do not pass along the Eustachian tube. Its function is to equalise the air pressure on  either side of the tympanic membrane.   The vestibular organ of the inner ear helps an animal maintain its posture and keep balanced by monitoring the  movement and position of the head. It consists of two structures - the semicircular canals and the otolith organs.   The semicircular canals (see diagram 15.10) respond to movement of the body. They tell an animal whether it is  moving up or down, left or right. They consist of three canals set in three different planes at right angles to each  other so that movement in any direction can be registered. The canals contain fluid and sense cells with fine hairs  that project into the fluid. When the head moves the fluid swirls in the canals and stimulates the hair cells. These  send nerve impulses along the vestibular nerve to the cerebellum.   Note that the semicircular canals register acceleration and deceleration as well as changes in direction but do not  respond to movement that is at a constant speed.   The otolith organs are sometimes known as gravity receptors. They tell you if your head is tilted or if you are  standing on your head. They consist of bulges at the base of the semi circular canals that contain hair cells that are  covered by a mass of jelly containing tiny pieces of chalk called otoliths (see diagram 15.10). When the head is  tilted, or moved suddenly, the otoliths pull on the hair cells, which produce a nerve impulse. This travels down the  vestibular nerve to the cerebellum. By coordinating the nerve impulses from the semicircular canals and otolith  organs the cerebellum helps the animal keep its balance.   • Receptors for touch, pressure, pain and temperature are found in the skin. Receptors in the muscles, tendons and  joints inform the brain of limb position.   • The olfactory organ in the nose responds to chemicals in the air i.e. smell.   • Taste buds on the tongue respond to a limited range of chemicals dissolved in saliva.   • The eyes are the organs of sight. Spherical eyeballs situated in orbits in the skull have walls composed of 3 layers.   • The tough outer sclera protects and holds the shape of the eyeball. At the front it becomes visible as the white of  the eye and the transparent cornea that allows light to enter the eye.   • The middle layer is the choroid. It most animals it absorbs stray light rays but in nocturnal animal it is reflective  to conserve light. At the front of the eye it becomes the iris with muscles to control the size of the pupil and  hence the amount of light entering the eye.   • The inner layer is the retina containing the light receptor cells: the rods for black and white vision in dim light  and the cones for colour and detailed vision. Nerve impulses generated by these cells leave the eye for the brain  via the optic nerve.   • The lens (with the cornea) helps focus the light rays on the retina. Muscles alter the shape of the lens to allow  near and far objects to be focussed.   • Aqueous humour fills the space immediately behind the cornea and keeps it in shape and vitreous humour, a  transparent jelly-like substance, fills the space behind the lens allowing light rays to pass through to the retina.   • The external pinna helps funnel sound waves into the ear and locate the direction of the sound. The sound waves  travel down the external ear canal to the eardrum or tympanic membrane causing it to vibrate. This vibration  is transferred to the auditory ossicles of the middle ear which themselves transfer it to the inner ear. Here  receptors in the cochlea respond by generating nerve impulses that travel to the brain via the auditory (acoustic)  nerve.   • The Eustachian tube connects the middle ear with the pharynx to equalise air pressure on either side of the  tympanic membrane.   • The vestibular organ of the inner ear is concerned with maintaining balance and posture. It consists of the  semicircular canals and the otolith organs.   1. Where are the organs that sense pain, pressure and temperature found?   3. Match the words in the list below with the following descriptions.   optic nerve I choroid I cornea I aqueous humor I retina I cones I iris I vitreous humour I sclera I lens   f) This layer of the eyeball reflects light in nocturnal animals like the cat.   g) This is the transparent window at the front of the eye.   h) This constricts in bright light to reduce the amount of light entering the eye.  i) The light rays are focused on here by the lens and cornea,  j) The chamber in front of the lens is filled with this.   4. Add the following labels to the diagram of the ear below.   pinna I Eustachian tube I cochlea I tympanic membrane I external ear canal I ear ossicles I semicircular canals   7. What is the role of the semicircular canals?  Senses Test Yourself Answers   • http://en.wikipedia.org/wiki/Sense Wikipedia. The old faithful. You can explore here to your hearts desire.  Try 'eye', 'ear', 'taste' etc. but also 'equilibrioception', and 'echolocation'.   • http://www.bbc.co.uk/science/humanbody/body/factfiles/smell/smell_ani_f5.swfBBC Science and  Nature. BBC animation of (human) olfactory organ and smelling.   • http://www.bbc.co.uk/science/humanbody/body/factfiles/taste/taste_ani_f5.swfBBC Science. BBC  animation of (human) taste buds and tasting.   • http://www.bishopstopford.com/faculties/science/arthur/Eye%20Drag%20%26%20Drop.swfEye Diagram.  A diagram of the eye to label and test your knowledge.   • http://www.bbc.co.uk/science/humanbody/body/factfiles/hearing/hearing_animation.shtml BBC on  Hearing. BBC animation of hearing. Well worth looking at.   • http://www.wisc-online.com/objects/index_tj. asp ?objid=AP1502 Ear Animation. Another great animation of  the ear and hearing.   • http://www.bbc.co.uk/science/humanbody/body/factfiles/balance/balance_ani_f5.swf BBC Balance  Animation. An animation of the action of the otolith organ (called macula in this animation)   1. Where are the organs that sense pain, pressure and temperature found?   The organs that sense pain, pressure and temperature are found in the dermis of the skin.   The olfactory organ in the nose responds to chemicals in the air.   3. Match the words in the list below with the following descriptions.   optic nerve I choroid I cornea I aqueous humor I retina I cones I iris I vitreous humour I sclera I lens   d) Carries nerve impulses from the retina to the brain - Optic nerve   e) The chamber behind the lens is filled with this - Vitreous humour   f) This layer of the eyeball reflects light in nocturnal animals like the cat - Choroid   g) This is the transparent window at the front of the eye. Cornea   h) This constricts in bright light to reduce the amount of light entering the eye - Iris  i) The light rays are focused on here by the lens and cornea - Retina  j) The chamber in front of the lens is filled with this - Aqueous humour   4. Add the following labels to the diagram of the ear below.   pinna I Eustachian tube I cochlea I tympanic membrane I external ear canal I ear ossicles I semicircular canals   The Eustachian tube equalises air pressure on either side of the tympanic membrane.   The ear ossicles transmit vibrations across the middle ear to the inner ear.  7. What is the role of the semicircular canals?   The semicircular canals respond to movement of the head including acceleration and slowing down.   • The position of the main endocrine glands in the  body   • The main hormones produced by the two parts of  the pituitary gland and their effects on the body   • The main hormones produced by the pineal,  thyroid, parathyroid and adrenal glands, the  pancreas, ovary and testis and their effects on the  body   • The homeostatic mechanisms that allow an animal  to control its body temperature, water balance,  blood volume and acid/base balance   endocrine systems both work together to bring about this adaptation. In general the nervous system responds rapidly   to short-term changes by sending electrical impulses along nerves and the endocrine system brings about longer-term   adaptations by sending out chemical messengers called hormones into the blood stream.   For example, think about what happens when a male and female cat meet under your bedroom window at night. The  initial response of both cats may include spitting, fighting and spine tingling yowling - all brought about by the  nervous system. Fear and stress then activates the adrenal glands to secrete the hormone adrenaline which increases  the heart and respiratory rates. If mating occurs, other hormones stimulate the release of ova from the ovary of the  female and a range of different hormones maintains pregnancy, delivery of the kittens and lactation.   Hormones are chemicals that are secreted by endocrine glands. Unlike exocrine glands (see chapter 5), endocrine  glands have no ducts, but release their secretions directly into the blood system, which carries them throughout the  body. However, hormones only affect the specific target organs that recognize them. For example, although it is  carried to virtually every cell in the body, follicle stimulating hormone (FSH), released from the anterior pituitary  gland, only acts on the follicle cells of the ovaries causing them to develop.   A nerve impulse travels rapidly and produces an almost instantaneous response but one that lasts only briefly. In  contrast, hormones act more slowly and their effects may be long lasting. Target cells respond to minute quantities of  hormones and the concentration in the blood is always extremely low. However, target cells are sensitive to subtle  changes in hormone concentration and the endocrine system regulates processes by changing the rate of hormone  secretion.   The main endocrine glands in the body are the pituitary, pineal, thyroid, parathyroid, and adrenal glands, the  pancreas, ovaries and testes. Their positions in the body are shown in diagram 16.1.   The pituitary gland is a pea-sized structure that is attached by a stalk to the underside of the cerebrum of the brain  (see diagram 16.2). It is often called the "master" endocrine gland because it controls many of the other endocrine  glands in the body. However, we now know that the pituitary gland is itself controlled by the hypothalamus. This  small but vital region of the brain lies just above the pituitary and provides the link between the nervous and  endocrine systems. It controls the autonomic nervous system, produces a range of hormones and regulates the  secretion of many others from the pituitary gland (see Chapter 7 for more information on the hypothalamus).   The pituitary gland is divided into two parts with different functions - the anterior and posterior pituitary (see  diagram 16.3).   Diagram 16.2 - The position of the pituitary gland and hypothalamus   The anterior pituitary gland secretes hormones that regulate a wide range of activities in the body. These include:   3. Follicle stimulating hormone (FSH) that stimulates the development of the follicles of the ovaries. These  then secrete oestrogen (see chapter 6).   The pineal gland is found deep within the brain (see diagram 16.4). It is sometimes known as the 'third eye" as it  responds to light and day length. It produces the hormone melatonin, which influences the development of sexual  maturity and the seasonality of breeding and hibernation.   The thyroid gland is situated in the neck, just in front of the windpipe or trachea (see diagram 16.5). It produces the  hormone thyroxine, which influences the rate of growth and development of young animals. In mature animals it  increases the rate of chemical reactions in the body.   Thyroxine consists of 60% iodine and too little in the diet can cause goitre, an enlargement of the thyroid gland.  Many inland soils in New Zealand contain almost no iodine so goitre can be common in stock when iodine  supplements are not given. To add to the problem, chemicals called goitrogens that occur naturally in plants like  kale that belong to the cabbage family, can also cause goitre even when there is adequate iodine available.   The parathyroid glands are also found in the neck just behind the thyroid glands (see diagram 16.5). They produce  the hormone parathormone that regulates the amount of calcium in the blood and influences the excretion of  phosphates in the urine.   The adrenal glands are situated on the cranial surface of the kidneys (see diagram 16.6). There are two parts to this  endocrine gland, an outer cortex and an inner medulla.   1. Aldosterone that regulates the concentration of sodium and potassium in the blood by controlling the  amounts that are secreted or reabsorbed in the kidney tubules.   2. Cortisone and hydrocortisone (Cortisol) that have complex effects on glucose, protein and fat metabolism.  In general they increase metabolism. They are also often administered to animals to counteract allergies and  for treating arthritic and rheumatic conditions. However, prolonged use should be avoided if possible as they  can increase weight and reduce the ability to heal.   3. Male and female sex hormones similar to those secreted by the ovaries and testes.   The hormones secreted by the adrenal cortex also play a part in "general adaptation syndrome" which occurs in  situations of prolonged stress.   The adrenal medulla secretes adrenalin (also called epinephrine). Adrenalin is responsible for the so-called flight  fight, fright response that prepares the animal for emergencies. Faced with a perilous situation the animal needs to  either fight or make a rapid escape. To do either requires instant energy, particularly in the skeletal muscles.  Adrenaline increases the amount of blood reaching them by causing their blood vessels to dilate and the heart to beat  faster. An increased rate of breathing increases the amount of oxygen in the blood and glucose is released from the  liver to provide the fuel for energy production. Sweating increases to keep the muscles cool and the pupils of the eye  dilate so the animal has a wide field of view. Functions like digestion and urine production that are not critical to  immediate survival slow down as blood vessels to these parts constrict.   Note that the effects of adrenalin are similar to those of the sympathetic nervous system.   In most animals the pancreas is an oblong, pinkish organ that lies in the first bend of the small intestine (see  diagram 16.7). In rodents and rabbits, however, it is spread thinly through the mesentery and is sometimes difficult  to see.   Most of the pancreas acts as an exocrine gland producing digestive enzymes that are secreted into the small  intestine. The endocrine part of the organ consists of small clusters of cells (called Islets of Langerhans) that secrete  the hormone insulin. This hormone regulates the amount of glucose in the blood by increasing the rate at which  glucose is converted to glycogen in the liver and the movement of glucose from the blood into cells.   In diabetes mellitus the pancreas produces insufficient insulin and glucose levels in the blood can increase to a  dangerous level. A major symptom of this condition is glucose in the urine.   The ovaries, located in the lower abdomen, produce two important sex hormones.   1 . The follicle cells, under the influence of FSH (see the pituitary gland above), produce oestrogen, which  stimulates the development of female sexual characteristics - the mammary glands, generally smaller build of  female animals etc. It also stimulates the thickening of the lining of the uterus in preparation for pregnancy  (see chapter 13).   2. Progesterone is produced by the corpus luteum, the endocrine gland that develops in the empty follicle  following ovulation (see chapter 13). It promotes the further preparation of the uterine lining for pregnancy  and prevents the uterus contracting until the baby is born.   Cells around the sperm producing ducts of the testis produce the hormone testosterone. This stimulates the  development of the male reproductive system and the male sexual characteristics - generally larger body of male  animals, mane in lions, tusks in boars, etc.   • Hormones are chemicals that are released into the blood by endocrine glands i.e. Glands with no ducts.  Hormones act on specific target organs that recognize them.   • The main endocrine glands in the body are the hypothalamus, pituitary, pineal, thyroid, parathyroid and  adrenal glands, the pancreas, ovaries and testes.   • The hypothalamus is situated under the cerebrum of the brain. It produces or controls many of the hormones  released by the pituitary gland lying adjacent to it.   • The pituitary gland is divided into two parts: the anterior pituitary and the posterior pituitary.   • Follicle stimulating hormone (FSH) that stimulates the development of ova   • Luteinising hormone (LH) that stimulates the development of the corpus luteum   • Antidiuretic hormone (ADH) that regulates water loss and raises blood pressure   • The pineal gland in the brain produces melatonin that influences sexual development and breeding cycles.   • The thyroid gland located in the neck, produces thyroxine, which influences the rate of growth and  development of young animals. Thyroxine consists of 60% iodine. Lack of iodine leads to goitre.   • The parathyroid glands situated adjacent to the thyroid glands in the neck produce parathormone that regulates  blood calcium levels and the excretion of phosphates.   • The adrenal gland located adjacent to the kidneys is divided into the outer cortex and the inner medulla.   • Aldosterone that regulates the blood concentration of sodium and potassium   • Cortisone and hydrocortisone that affect glucose, protein and fat metabolism   • The adrenal medulla produces adrenalin responsible for the flight, fright, fight response that prepares animals  for emergencies.   • The pancreas that lies in the first bend of the small intestine produces insulin that regulates blood glucose levels.   • The ovaries are located in the lower abdomen produce 2 important sex hormones:   • The follicle cells of the developing ova produce oestrogen, which controls the development of the mammary  glands and prepares the uterus for pregnancy.   • The corpus luteum that develops in the empty follicle after ovulation produces progesterone. This hormone  further prepares the uterus for pregnancy and maintains the pregnancy.   • The testes produce testosterone that stimulates the development of the male reproductive system and sexual  characteristics.   Animals can only survive if the environment within their bodies and their cells is kept constant and independent of  the changing conditions in the external environment. As mentioned in module 1.6, the process by which this stability  is maintained is called homeostasis. The body achieves this stability by constantly monitoring the internal conditions  and if they deviate from the norm initiating processes that bring them back to it. This mechanism is called feedback  control. For example, to maintain a constant body temperature the hypothalamus monitors the blood temperature and  initiates processes that increase or decrease heat production by the body and loss from the skin so the optimum  temperature is always maintained. The processes involved in the control of body temperature, water balance, blood  loss and acid/base balance are summarized below.   The biochemical and physiological processes in the cell are sensitive to temperature. The optimum body temperature  is about 37D C [99D F] for mammals, and about 40D C [104D F] for birds. Biochemical processes in the cells,  particularly in muscles and the liver, produce heat. The heat is distributed through the body by the blood and is lost  mainly through the skin surface. The production of this heat and its loss through the skin is controlled by the  hypothalamus in the brain which acts rather like a thermostat on an electric heater. .   (a) When the body temperature rises above the optimum, a decrease in temperature is achieved by:   • Expansion of the blood vessels near the skin surface so heat is lost to the air.   (b) When the body temperature falls below the optimum, an increase in temperature can be achieved by:   • Moving to a heat source e.g. in the sun, out of the wind.   • Making the hair stand on end by contraction of the hair erector muscles or fluffing of the feathers so there is an  insulating layer of air around the body   • Constricting the blood vessels near the skin surface so heat loss to the air is decreased   The concentration of the body fluids remains relatively constant irrespective of the diet or the quantity of water taken  into the body by the animal. Water is lost from the body by many routes (see module 1.6) but the kidney is the main  organ that influences the quantity that is lost. Again it is the hypothalamus that monitors the concentration of the  blood and initiates the release of hormones from the posterior pituitary gland. These act on the kidney tubules to  influence the amount of water (and sodium ions) absorbed from the fluid flowing along them.   (a) When the body fluids become too concentrated and the osmotic pressure too high, water retention in the kidney  tubules can be achieved by:   • An increased production of antidiuretic hormone (ADH) from the posterior pituitary gland, which causes more  water to be reabsorbed from the kidney tubules.   • A decreased blood pressure in the glomerulus of the kidney results in less fluid filtering through into the kidney  tubules so less urine is produced.   (b) When the body fluids become too dilute and the osmotic pressure too low, water loss in the urine can be achieved  by:   • A decrease in the secretion of ADH, so less water is reabsorbed from the kidney tubules and more diluted urine is  produced.   • An increase in the blood pressure in the glomerulus so more fluid filters into the kidney tubule and more urine is  produced.   • An increase in sweating or panting that also increases the amount of water lost.   Another hormone, aldosterone, secreted by the cortex of the adrenal gland, also affects water balance indirectly. It  does this by increasing the absorption of sodium ions (Na-) from the kidney tubules. This increases water retention  since it increases the osmotic pressure of the fluids around the tubules and water therefore flows out of them by  osmosis.   Loss of blood or body fluids leads to decreased blood volume and hence decreased blood pressure. The result is that  the blood system fails to deliver enough oxygen and nutrients to the cells, which stop functioning properly and may  die. Cells of the brain are particularly vulnerable. This condition is known as shock.   If blood loss is not extreme, various mechanisms come into play to compensate and ensure permanent tissue damage  does not occur. These mechanisms include:   • Blood vessels in the skin and kidneys constrict to reduce the total volume of the blood system and hence retain  blood pressure.   • Antidiuretic hormone (ADH) is released by the posterior pituitary gland. This increases water re-absorption in the  collecting ducts of the kidney tubules so concentrated urine is produced and water loss is reduced. This helps  maintain blood volume.   • Loss of fluid causes an increase in osmotic pressure of the blood. Proteins, mainly albumin, released into the  blood by the liver further increase the osmotic pressure causing fluid from the tissues to be drawn into the blood  by osmosis. This increases blood volume.   • Aldosterone, secreted by the adrenal cortex, increases the absorption of sodium ions (Na-) and water from the  kidney tubules. This increases urine concentration and helps retain blood volume.   If blood or fluid loss is extreme and the blood volume falls by more than 15-25%, the above mechanisms are unable  to compensate and the condition of the animal progressively deteriorates. The animal will die unless a vet  administers fluid or blood.   Biochemical reactions within the body are very sensitive to even small changes in acidity or alkalinity (i.e. pH) and  any departure from the narrow limits disrupts the functioning of the cells. It is therefore important that the blood  contains balanced quantities of acids and bases.   The normal pH of blood is in the range 7.35 to 7.45 and there are a number of mechanisms that operate to maintain  the pH in this range. Breathing is one of these mechanisms.   Much of the carbon dioxide produced by respiration in cells is carried in the blood as carbonic acid. As the amount  of carbon dioxide in the blood increases the blood becomes more acidic and the pH decreases. This is called acidosis  and when severe can cause coma and death. On the other hand, alkalosis (blood that is too alkaline) causes over  stimulation of the nervous system and when severe can lead to convulsions and death.   (a) When vigorous activity generating large quantities of carbon dioxide causes the blood to becomes too acidic it  can be counteracted in two ways:   • By the rapid removal of carbon dioxide from the blood by deep, panting breaths  By the secretion of hydrogen ions (H+) into the urine by the kidney tubules.   (b) When over breathing or hyperventilation results in low levels of carbon dioxide in the blood and the blood is too  alkaline, various mechanisms come into play to bring the pH back to within the normal range. These include:   • A reduction in the amount of hydrogen ions (H+) secreted into the urine.   Homeostasis is the maintenance of constant conditions within a cell or animal's body despite changes in the external  environment.   Thebody temperature of mammals and birds is maintained at an optimum level by a variety of heat regulation  mechanisms. These include:   • Contraction of the erector muscles so hairs and feathers stand up to form an insulating layer,   • adjusting the amount of water lost through sweating or panting.   • drawing fluid from the tissues into the blood by increasing the osmotic pressure of the blood.   Animals maintain the acid/base balance or pH of the blood by:   • Adjusting the rate of breathing and hence the amount of C02 removed from the blood.   3. List 3 ways in which animals keep their body temperature constant when the weather is hot   4. How does the kidney compensate when an animal is deprived of water to drink   5. After moderate blood loss, several mechanisms come into play to increase blood pressure and make up blood  volume. 3 of these mechanisms are:   6. Describe how panting helps to reduce the acidity of the blood  Test Yourself Answers   • http://www.zerobio.com/drag_oa/endo.htm A drag and drop hormone and endocrine organ matching  exercise.   • http://en.wikipedia.org/wiki/Endocrine_system Wikipedia. Much, much more than you ever need to know  about hormones and the endocrine system but with a bit of discipline you can glean lots of useful information  from this site.   Examples of homeostasis in an animal 's body are the maintenance of a constant internal temperature despite  fluctuations in the air temperature, mechanisms that keep the concentration of the internal fluids constant  despite changes in the diet or fluid intake, mechanisms that compensate for blood loss and mechanisms that  maintain a constant internal acid/base balance or pH.   3. List 3 ways in which animals keep their body temperature constant when the weather is hot.   Animals keep their body temperature constant when the weather is hot by keeping out of the sun, keeping  activity to the minimum, sweating, panting and dilation of the surface blood vessels to increase heat loss to the  environment.   4. How does the kidney compensate when an animal is deprived of water to drink?   The kidney compensates when an animal is deprived of water to drink through the action of the hormones  antidiuretic hormone (ADH) and aldosterone, both of which act to increase the amount of water reabsorbed  from the kidney tubule and increase the concentration of the urine produced. When the blood pressure  decreases as occurs when fluid intake is insufficient, the reduced flow of blood through the glomerulus of the  kidney tubule also reduces the amount of fluid filtered into the tubule so less urine is produced.   5. After moderate blood loss, several mechanisms come into play to increase blood pressure and make up blood  volume. 3 of these mechanisms are:   After moderate blood loss, several mechanisms come into play to increase blood pressure and make up blood  volume. These mechanisms include: thirst, constriction of blood vessels in the skin and kidneys, increasing  heart rate, secretion of antidiuretic hormone and aldosterone, as well as drawing fluid from the tissues into  the blood by increasing the osmotic pressure of the blood.   6. Describe how panting helps to reduce the acidity of the blood.   Panting helps to reduce the acidity of the blood by blowing out carbon dioxide. Carbon dioxide dissolved in  the blood makes the blood acidic so removal of this carbon dioxide reduces the acidity of the blood.    The final compartment of the stomach of ruminants.  This the 'true' stomach where muscular walls churn  the food and gastric juice is secreted    A gland that produces secretions that make up the liquid portion of semen    The movement of substances across a membrane against  the concentration gradient. Requires energy    Connective tissue in which the cells are filled with fat or lipid    The long canal from the mouth to the anus through which food passes as it is digested and absorbed    An air sac in the lung, where gas exchange takes place    Enzymes that split carbohydrates like starch and glycogen into monosaccharides like glucose    A condition involving a low number of red blood cells or haemoglobin in blood    A pair of muscles that work together such that as one contracts the other relaxes and vice versa.    Nearer to the front of the body (usually used in human anatomy)    Proteins made in response to a stimulating molecule called an antigen. The basis for the immune response    A hormone produced by posterior pituitary gland that stimulates water  reabsorption from the kidney tubule    The opening at the lower end of the rectum through which solid waste is eliminated.    The watery fluid that fills the anterior chamber of the eye    The point of contact between bones. Where they move against each other    The watery fluid that fills the anterior chamber of the eye    The point of contact between bones. Where they move against each other.  A joint. Point of contact between 2 bones    A valve that prevents blood flow backwards from ventricle to atrium    The part of the vertebrate nervous system that innervates smooth and cardiac muscle and  glandular tissues and governs involuntary actions. Consists of the sympathetic nervous  system and the parasympathetic nervous system.    A long extension from the neuron that carries nerve impulses away from the cell body    A synovial joint where rounded end of one bone fits into cup-shaped depression of another    The part of the feather that sticks out of the shaft    The parts of a feather that grow out of the barbs. They have hooks and rolled edges to lock the barbs together    The muscle that extends from the shoulder to the elbow responsible for flexing the forearm    An alkaline secretion from liver that helps break down fats into small droplets    The placement of the eyes such that both see the same wide area but from slightly different angles    The two-part Latinized name of a species, consisting of genus and species names    A hollow ball of cells that develop from the fertilized ovum    The area of retina at end of optic nerve where there are no receptor cells    The pressure of blood on the walls of the blood vessels    The part of the brain just above the spinal cord. Contains the medulla oblongata    A thickening of the skin or growth of new bone tissue in and around a fracture    An organic compound containing carbon, hydrogen and oxygen. Made up of sugar subunits    The muscle that makes up the wall of the heart. Striated branched fibres    The modified premolars and molars in carnivores that slice against each other like scissors for shearing flesh and bone.    Dense connective tissue found at ends of long bones, in trachea, ear pinna. Also forms the skeleton of the foetus    A condition in which the lens becomes cloudy resulting in blurred vision.    The large vein that collects blood from the body caudal to the heart    The process by which a cell reproduces itself. Two types: mitosis and meiosis    A substance produced by cell that is free in cytoplasm i.e. not enclosed by a membrane    The chemical pathway that produces energy in the cell It consumes a fuel, generally glucose, in the presence of oxygen    The part of the nervous system consisting of the brain and spinal cord    The part of the vertebrate hindbrain located dorsally; functions in unconscious coordination of movement and balance    The surface of the cerebrum; the largest and most complex part of the mammalian brain, containing sensory and motor  nerve cell bodies of the cerebrum    The fluid that circulates around and within brain and spinal cord    The dorsal portion of the brain composed of right and left hemispheres; the integrating center for memory, learning,  emotions    The hormone secreted by the placenta that prevents uterine contractions before labour  and prepares the mammary glands for lactation    One of the small dark staining bodies in the cell nucleus. Contains the DNA    The milky fluid found in the lacteals of the small intestine    The semi fluid mixture of partly digested food and digestive secretions in the stomach and small intestine    A hair like process projecting from a cell. Used to move cell substances along the surface of the cell    The coiled tube forming the portion of the inner ear that converts sound waves to nerve impulses    A protein that is the main organic constituent of connective tissue    The duct that carries both bile and pancreatic juice into the small intestine    The response that is elicited by a stimulus after training has taken place    A rounded protuberance at the ends of some bones where it forms an articulation with another bone    A light sensitive receptor in the retina that responds to colour    One of the 4 basic tissue types of the body. Binds and supports. Consists of cells and fibres in a matrixs    The transparent anterior layer of the eye through which the iris can be seen    A yellow endocrine gland formed in the empty ovarian follicle after ovulation    The bag-like structure at the base of the oesophagus in birds. In many birds it stores food before it enters the stomachA    Secretion produced by glands in the wall of the crop of in pigeons and doves Parents regurgitate it to feed their young    Disruption of the structure of a protein by heat, acids etc. to make it inactive    A nerve cell process that carries the nerve impulses towards the cell body    The formula that describes the numbers of the different kinds of teeth    The condition caused by under secretion of antidiuretic hormone (ADH). Symptom: excretion large amounts dilute urine    The condition caused by under secretion of insulin. Symptoms: raised blood glucose levels, glucose in urine    The dome shaped skeletal muscle separating the thoracic from the abdominal cavities    The space in the jaw in animals that have no (or reduced) canines    The phase of the heartbeat involving the relaxation of the ventricles    Blood pressure in the arteries between the passage of the pulses    A passive process of movement of molecules from a region of high concentration to one of low concentration    Terms that describe the locations of structures in relation to other structures or locations in the body    Farther away from the trunk of the body or point of origin    A chemical that reduces Antidiuretic hormone production and increases urine volume   The use of high frequency sound like sonar and radar by animals (i.e. whales and bats) to locate objects in the surrounding  environment   A muscle or gland that responds to a motor neuron impulse   The elimination of indigestible waste products from the body  A compound that separates into charged particles or ions   A microscope that focuses an electron beam through a specimen, resulting in resolving power a thousandfold greater than  that of a light microscope   Any substance that cannot be broken down to any other substance   Embryo The young of any organism in an early stage of development   Emulsification The breakdown of large fat particles to smaller ones in the presence of bile   Endocrine gland A ductless gland that secretes hormones into the blood   The network of membranous channels running through the cytoplasm of cells   Enzyme A substance that increases the speed of a chemical reaction   A white blood cell with granules in the cytoplasm  The thin outer layer of the skin   Epididymis The organ composed of convoluted tubules that lies on the border of the testis Where sperm mature   Epiglottis The cartilage on the top of the larynx that closes the windpipe during swallowing   Epiphyseal line The remnant of epiphyseal plate at end of long bone   Epiphyseal plate The cartilaginous plate at the end of a long bone where bone growth occurs   Epithelial tissue Tissue that forms outer part of skin, lines blood vessels, hollow organs and passages in the body   The 10 amino acids that can not be made by animals and must be acquired in the diet   Eustachian tube The passage connecting middle ear to pharynx. Equalises air pressure in middle and outer ear   Evolution All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today   Extension Bending of a joint so that the angle between the bones increases    A slender tube through which eggs pass from an ovary to the uterus    Biological compounds consisting of three fatty acids linked to one glycerol molecule    The sequence of events where information about the status of a situation is continually fed back to the central control    The joint between the metacarpals or metatarsals and the phalanges in horse    Hormone secreted by anterior pituitary gland. Stimulates development of ovarian follicle    A hole in a bone for passage of vessels or nerves    The hole at the base of the skull for passage of the spinal cord    The area of the retina of greatest concentration of cone cells. Area of sharpest vision    The enlarged large intestine and caecum occupied by cellulose digesting micro-organisms   Gas exchange The process in which oxygen from inhaled air is transferred into the blood and   The digestive secretion produced by glands in the wall of the stomach  A biological unit of heredity   The period of foetal development inside the uterus  An encircling or arching arrangement of bones   The second part of the stomach of birds. In seed eating birds it contains pebbles and its muscular walls help grind the  food   A synovial joint with flat articulating surfaces that permits limited movements e.g. between carpals and tarsals    A molecule that combines with three fatty acid molecules to form a fat or oil    A cell organelle concerned with packaging, processing and secretion of organic molecules    A hormone secreted by the anterior pituitary gland. Stimulates growth, particularly of the skeleton    The percentage of blood made up of red blood cells. Also called packed cell volume (PCV)    Pigment containing iron in red blood cells that allows them to carry oxygen    A naturally occurring anticoagulant. Also used in laboratory tests for heavy metals    The blood vessel that carries blood from the intestines to the liver    A synovial joint that allows movement in only one plane e.g. elbow    A substance secreted from white cells and platelets that is involved in the inflammatory response    The bone of the upper forearm between the scapula and the radius and ulna    Having an osmotic pressure higher than a solution with which it is compared    Having an osmotic pressure lower than a solution with which it is compared    The chisel-shaped 'biting off teeth at the front of the mouth    When ovulation is stimulated by mating as in cat and rabbit    Towards the lower part of the body. Not used in animals except, perhaps, higher apes    The attachment of a muscle tendon to a bone that moves    Digestive secretion produced by glands in the lining of the small intestine    Having an osmotic pressure equal to that of a solution with which it is compared    Part of the gut consisting of the colon, caecum, rectum and anal canal    Transparent part of the eye that helps focus light rays on the retina    The hormone from the anterior pituitary gland that stimulates ovulation and development of corpus    The substance of a tissue in which the more specialised structures are embedded    The type of cell division for production of gametes. Halves the number of chromosomes    The distant spread of disease especially a malignant tumour from its site of origin    The five bones of the foot that connect the "ankle" to the toes    A solid contracting strand in the cytoplasm of cells that brings about cell contraction.    A hollow rod of protein in the cytoplasm of all eukaryotic cells and in cilia, flagella, and the cytoskeleton    The microscopic fingerlike projections from the membrane of the cells covering the villi of the small intestine    The cavity in the skull between the eardrum and inner ear housing the auditory ossicles    The volume of air inspired or expired during a minute of normal tidal breathing    The largest leukocyte. It is phagocytic and has no granules in the cytoplasm    The solid mass of cells produced by successive divisions of the fertilized ovum    Control in which the stimulus initiates actions that reverse or reduce the stimulus    Molecules released at a synapse to transmit the nerve impulse from one neuron to the next    A fat or triglyceride. Biological compound consisting of three fatty acids linked to one glycerol molecule    White blood cell with granules in the cytoplasm involved in phagocytosis    The spherical or oval body in the cell that contains the DNA    A chemical substance in food that provides energy or assists various body processes    Part of the modified stomach of ruminants with a folded inner surface    Teeth in which the root opening remains wide. They grow continuously e.g. the incisors of rabbits and rats    The nerve carrying impulses from the retina of the eye to the brain    A compound that contains carbon and hydrogen e.g. carbohydrates, lipids and proteins    The attachment of a muscle to a bone that does not move    The movement of water molecules across a semi permeable membrane from an area of high water  concentration to an area of low water concentration    The pressure required to prevent water moving across a semi permeable membrane by osmosis    A particle of calcium carbonate embedded in the membrane of the otolith organ of the inner ear    The series of events in the ovary associated with the maturation of the ovum    The release of the ovum from the mature follicle of the ovary    The hormone from the posterior pituitary gland. Stimulates milk "let down"    The organ lying along the caudal margin of the stomach. Has endocrine and exocrine functions    One of the two parts of the autonomic nervous system. Concerned with normal "at rest" activities    One of four small endocrine glands on the dorsal surface of the thyroid gland    The caudal portion of the abdominal cavity. Contains the bladder, colon and reproductive structures    The bony structure formed by the hip bones, sacrum and coccygeal bones    The structure formed by the two hip bones, sacrum and coccyx    The small cavity between the two layers of the pericardial membranes    The part of the nervous system composed of the cranial and spinal nerves    The membrane that lines the abdominal cavity and covers the abdominal organ    Large clusters of lymph nodules found in the wall of the small intestine    A symbol that indicates the acidity or alkalinity of a solution    The making of organic molecules by plants using energy from the sun    The science that deals with the functions of an organism and its parts    The endocrine gland lying under the caudal surface of the brain attached to the hypothalamus by a stalk    A synovial joint where a peg of bone articulates with a ring of bone as in the joint between the atlas and  axis    The special structure through which the exchange of materials between the foetus and mother occurs    Locomotion involving placing the whole surface of the foot on the ground as in humans and bears    Membranes that cover the lungs and line the walls of the chest and diaphragm    'Cold-blooded' animals whose body temperature varies, to a large extent depending on the environment    An organic compound consisting of carbon, hydrogen, oxygen and nitrogen. Made up of amino acids    The series of waves of high pressure blood passing along an artery    The hole in the centre of the iris of the eye    The ring of smooth muscle between the stomach and the small intestine    The shorter bone of the forelimb between the humerus and the "wrist"    A specialized cell that responds to specific sensory stimuli such as touch, pressure, light etc.    Bone marrow found in the spaces of spongy bone.Makes red blood cells    Bending of light as it passes from one medium to another    The hormone secreted by the placenta and ovaries that eases the joint between the right and left pelvis  and dilates the cervix for birth    The part of the modified stomach of ruminants with honeycomb of raised folds on its inner surface    The photoreceptor in the retina, specialized for vision in dim light    The triangular bone formed from fused sacral vertebrae. Located between the two hipbones    An exocrine gland in the dermis of the skin associated with a hair follicle    A characteristic that develops at sexual maturity, e.g. large body size of males, manes in lions    The valve guarding the entrance to the aorta or the pulmonary artery    The tightly coiled duct in the testis where sperm are produced    A membrane that allows some substances to cross more easily than others    A neuron that carries a nerve impulse towards the central nervous system    Reduced cardiac output resulting in failure to deliver adequate oxygen and nutrients to the body    An air cavity in a bone especially in the bones of the face or skull    Tissue specialized for contraction with striated fibres. Attached to the bones of the skeleton    The long tube of the gut that begins at the stomach and ends at the large intestine    A measure of the density of a liquid or solid, as compared with that of water.    The tube that carries sperm from the epididymis to the urethra. Also called the vas deferens    The structure in the male reproductive system attached to the testis that carries the vas deferens, arteries, veins, etc.    A ring-like muscle that controls movement along a body passage or orifice    The large lymphatic organ near the stomach that stores blood and produces lymphocytes    The inner layer of bone; found at the ends of long bones less dense than compact bone    The joint between the femur and the tibia on the hind leg    Any change in the environment capable of initiating a nerve impulse    The large baglike part of the gut between the oesophagus and the small intestine    One of the two subdivisions of the autonomic nervous system concerned with reacting to emergency situations    An association of organs that have a common function, e.g. digestive system, respiratory system    The blood circulation from the left ventricl through the aorta to all the organs of the body and back to the heart    A tough cord of fibrous connective tissue that connects muscles to bones    The hormone produced by the cells between the seminiferous tubules of the testis    The part of the body between the neck and the diaphragm    The organ dorsal to the sternum that is essential to the immune response    The endocrine gland with lobes on either side of the trachea    The volume of air breathed in or out in any one "at rest" breath    Plasma that has left the capillaries and flowed into the spaces between the cells of the tissues; also known as intercellular  fluid or interstitial fluid    The sum of the tidal volume, inspiratory reserve, expiratory reserve and residual volume of the lungs    The muscle that extends from the shoulder to the elbow responsible for extending the forearm    A biological compound consisting of three fatty acids linked to one glycerol molecule. A fat    The part of the body to which the fore and hind limbs are attached    The thin transparent membrane of connective tissue between the external ear, canal and the middle ear. Also called the  eardrum    The longer bone of the forelimb between the humerus and the "wrist"    The cord containing arteries and vein that attaches the foetus to the placenta    The soluble excretory product produced when excess amino acids (from proteins) are broken down by the body    One of two tubes that connect the kidney with the bladder    The duct from the bladder to the exterior of the body    An insoluble excretory product produced when excess amino acids(from proteins) are broken down by the body    The muscular, tubular organ in the female where sperm are deposited during copulation    The cranial nerve that controls the muscles that bring about swallowing, the muscles of the heart, airways, lungs, stomach  and intestines    The flat part of a feather emerging from the shaft; there are two vanes per feather    The duct that conducts the sperm from the epididymis to the urethra    The decrease in size of the channel down a blood vessel    The increase in size of the channel down a blood vessel 
AnatomyAndPhysiologyOfAnimals-Wikibooks AnatomyAndPhysiologyOfAnimals_djvu.txt