FIGURE 13.27 • Photomicrograph of the coronary artery. This photomicrograph of a cross section of the coronary artery obtained from adult human shows all three vascular tunics similar to those in muscular arteries. The subendothelial layer of the tunica intima (TI) is considerably thicker because of the aging process than a comparable muscular artery. The internal elastic membrane (IEM) is visible at the border with the tunica media (TM), which is also thicker than in other muscular-type arteries. Connective tissue of the tunica adventitia (TA) is loosely arranged and contains peripherally positioned longitudinal bundles of collagen fibers. There is an artificial separation between cardiac muscle (CM) and tunica adventitia. 175.
Lymphatic capillaries are a unique part of the circulatory system, forming a network of small vessels within the tissues. Because of their greater permeability, lymphatic capillaries are more effective than blood capillaries in removing protein-rich fluid from the intercellular spaces. Once the collected fluid enters the lymphatic vessel, it is called lymph. Lymphatic vessels also serve to convey proteins and lipids that are too large to cross the fenestrations of the absorptive capillaries in the small intestine.
Before lymph is returned to the blood, it passes through lymph nodes, where it is exposed to the cells of the immune system. Thus, the lymphatic vessels serve not only as an adjunct to the blood vascular system but also as an integral component of the immune system.
Lymphatic capillaries are essentially tubes of endothelium that, unlike the typical blood capillary, lack a continuous basal lamina. This incomplete basal lamina can be correlated with their high permeability. Anchoring filaments extend between the incomplete basal lamina and the perivascular collagen. These filaments may help maintain the patency of the vessels during times of increased tissue pressure such as in inflammation.
FIGURE 13.28 • Photomicrograph of the great saphenous vein. This photomicrograph shows a section through the wall of the greater saphenous vein. Tunica intima is usually thicker than in the other medium size veins and is characterized by a presence of numerous longitudinal smooth muscle bundles (SM) separated by connective tissue fibers. Tunica media contains relatively thick layer of circularly arranged smooth muscles. Tunica adventitia is well developed and contains additional layers of smooth muscle fibers arranged in spiral, oblique, and longitudinal bundles. 380. (Courtesy of Dr. Joseph J. Maleszewski, Mayo Clinic, Rochester, MN.)
As lymphatic vessels become larger, the wall becomes thicker. The increasing thickness is because of connective tissue and bundles of smooth muscle. Lymphatic vessels possess valves that prevent backflow of the lymph, thus aiding unidirectional flow (Plate 35, page 438). There is no central pump in the lymphatic system. Lymph moves sluggishly, driven primarily by compression of the lymphatic vessels by adjacent skeletal muscles.