The terminal vessels of the lymphatic system consist of a widely distributed, closed-end network of highly permeable lymphatic capillaries. These lymphatic capillaries resemble blood capillaries, with two important differences: tight junctions are not present between endothelial cells, and fine filaments anchor lymphatic vessels to the surrounding connective tissue. With muscular contraction, these fine strands pull on the lymphatic vessels to open spaces between the endothelial cells and enable the entrance of protein and large particles into the lymphatic vessels. The lymphatic capillaries drain into larger vessels that finally enter the right and left subclavian veins, where they connect with the respective internal jugular veins.
Only cartilage, bone, epithelia, and tissues of the central nervous system lack lymphatic vessels. These vessels return the plasma capillary filtrate to the circulation. This task is accomplished by means of tissue pressure, and it is facilitated by intermittent skeletal muscle activity, lymphatic vessel contractions, and an extensive system of one-way valves. In this regard, lymphatic vessels resemble veins, although the larger lymphatic vessels do have thinner walls than do the corresponding veins, and they contain only a small amount of elastic tissue and smooth muscle.
The volume of fluid transported through the lymphatic vessels in 24 hours is approximately equal to the body’s total plasma volume. The lymphatic vessels return all of the proteins filtered back to the blood; these proteins account for approximately one fourth to half of the circulating plasma proteins in the blood. The lymphatic vessels are the only means by which the protein that leaves the vascular compartment can be returned to blood. Net backward diffusion of protein into the capillaries cannot occur against the large protein concentration gradient. If the protein were not removed by the lymph vessels, it would accumulate in interstitial fluid and act as an oncotic force that draws fluid from the blood capillaries and produces edema.
In addition to returning fluid and protein to the vascular bed, the lymphatic system filters the lymph at the lymph nodes and removes foreign particles such as bacteria. The
CHAPTER 17 Properties of the Vasculature largest lymphatic vessel, the thoracic duct, not only drains the lower extremities but also returns the protein lost through the permeable liver capillaries. Moreover, the thoracic duct carries substances absorbed from the gastrointestinal tract. The principal substance is fat, in the form of chylomicrons.
Lymph flow varies considerably. The flow from resting skeletal muscle is almost nil, and it increases during exercise in proportion to the degree of muscular activity. It is increased by any mechanism that enhances the rate of blood capillary filtration; such mechanisms include increased capillary pressure or permeability and decreased plasma oncotic pressure. When the volume of interstitial fluid exceeds the drainage capacity of the lymphatic vessels, or when the lymphatic vessels become blocked, interstitial fluid accumulates and gives rise to clinical edema.
Functional Anatomy of Coronary Vessels
The right and left coronary arteries arise at the root of the aorta behind the right and left cusps of the aortic valve, respectively. These arteries provide the entire blood supply to the myocardium. The right coronary artery supplies mainly the right ventricle and atrium. The left coronary artery, which divides near its origin into the anterior descending and the circumflex branches, supplies mainly the left ventricle and atrium. There is some overlap between the regions supplied by the left and right arteries. In humans, the right coronary artery is dominant (supplying most of the myocardium) in approximately 50% of individuals. The left coronary artery is dominant in another 20%, and the flow delivered by each main artery is approximately equal in the remaining 30%. The epicardial distribution of the coronary arteries and veins is illustrated in
Fig. 17.29