Unicellular organisms maintain their volume and composition through exchanges with the environment they inhabit (e.g., sea water). The billions of cells that constitute the human body must maintain their volume and composition as well, but their task is much more difficult. This challenge, as well as its solution, was first articulated by the French physiologist Claude Bernard (1813-1878). He recognized that although cells within the body cannot maintain their volume and composition through exchanges with the environment, they can do so through exchanges with the fluid environment that surrounds them (i.e., the extracellular fluid). Bernard referred to the extracellular fluid as the milieu intérieur (“the environment within”). He also recognized that the organ systems of the body are designed and function to maintain a constant milieu interieur or a “constant internal environment.” This in turn allows all cells to maintain their volume and composition through exchanges with the extracellular fluid as a result of membrane transport (see
Transport by the epithelial cells of the gastrointestinal tract, kidneys, and lungs are the body’s interface with the
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments external environment and control both the intake and excretion of numerous substances, as well as water. The cardiovascular system delivers nutrients to and removes waste products from the cells and tissues and keeps the extracellular fluid well mixed. Finally, the nervous and endocrine systems provide regulation and integration of these important functions.
To provide background for the study of all organ systems, this chapter presents an overview of the normal volume and composition of the body fluid compartments and describes how cells maintain their intracellular composition and volume. Included is a presentation on how cells generate and maintain a membrane potential, which is fundamental for understanding the function of excitable cells (e.g., neurons and muscle cells). Finally, because epithelial cells are so central to the process of regulating the volume and composition of the body fluids, the principles of solute and water transport by epithelial cells are also reviewed.
Definition and Volumes of Body Fluid Compartments
Water makes up approximately 60% of the body’s weight; variability among individuals is a function of the amount of adipose tissue. Because the water content of adipose tissue is lower than that of other tissue, increased amounts of adipose tissue reduce the fraction of water in the total body weight. The percentage of body weight attributed to water also varies with age. In newborns, it is approximately 75%. This decreases to the adult value of 60% by the age of 1 year.
Fig. 2.2, total body water is distributed between two major compartments, which are divided by the cell membrane.
The intracellular fluid (ICF) compartment is the larger compartment, and contains approximately two thirds of the total body water. The remaining third is contained in the extracellular fluid (ECF) compartment. Expressed as percentages of body weight, the volumes of total body water, ICF, and ECF are as follows:
Total body water = 0 6. × (body weight) ICF = 0 4. × (body weight) ECF = 0 2. × (body weight)
The ECF compartment is further subdivided into interstitial fluid and plasma. The ECF also includes fluid contained within bone and dense connective tissue, as well as the cerebrospinal fluid. The interstitial fluid surrounds the cells in the various tissues of the body and makes up three fourths of the ECF volume. Plasma is contained within the vascular compartment and represents the remaining fourth of the ECF. In some pathological conditions, additional fluid may accumulate in what is referred to as a third space. Third-space collections of fluid are part of the ECF; an aIn these and all subsequent calculations, it is assumed that 1 L of fluid (e.g., ICF and ECF) has a mass of 1 kg. Although, 1 L of the ICF and ECF has a mass of slightly more than 1 kg, this simplification allows conversion from measurements of body weight to volume of body fluids.
Total body water (TBW) 0.6 x body weight 42 L Extracellular fluid (ECF) 0.2 x body weight 14 L Intracellular fluid (ICF) 0.4 x body weight 28 L Cell membrane