Walls of Vessels

The walls of vessels have three layers:

• Tunica Intima: the innermost layer, including the epithelium
• Tunica Media: the muscular middle layer
• Tunica Adventitia: the outermost connective tissue layer

There are three types of arteries based on size and the characteristics of the tunica media. Each of these have a wall composed of the three layers:

• Larger arteries or Elastic arteries include the aorta and pulmonary arteries. Their main function is conveyance of blood. Systole causes distension of the elastic wall and recoil of the wall to help maintain blood pressure during diastole. Characteristics of these vessels are numerous concentric elastic lamellae of the tunica media interspersed with bundles of smooth muscle cells. Elastin can be stained with special stains. Elastic fibers in the media allow for the maintenance of blood pressure through the expansion and contraction of the vessel walls. The vasa vasorum, a network of small vessels that supplies the cells with larger vessels, is present in their adventitia.
• Medium arteries or Muscular arteries follow the elastic arteries. Through the controlled contraction of the walls, these arteries distribute the blood to different parts of the body according to regional needs. The media of muscle arteries is composed of many concentric layers of smooth muscle cells. Few elastic fibers. The internal elastic membrane marks the conventional boundary between intima and media. The external elastic membrane separates the media from the adventitia. The media and adventitia of muscular arteries are approximately equal in thickness.
• Small arteries may have up to 8 layers of smooth muscle in the tunica media, and may have an internal elastic membrane. Adventitia is very thin and inconspicuous.
• Capillaries are vessels of small diameter (4-10μm). The wall is reduced to an endothelium surrounded by a basement membrane, a few pericytes, and connective tissue. The narrow capillary lumen allows passage of red blood cells in a single file. At the level of the electron microscope, three different types of capillaries can be resolved based on the morphology of their endothelial layer:
• Continuous Capillaries are continuous with endothelium and less permeable. Present in muscles, lung, connective tissue, and skin.
• Fenestrated Capillaries have gaps between the endothelial cells, but the basement is still continuous. These are found in the renal glomeruli and endocrine glands. They allow a high degree of permeability for water and water-soluble molecules, as well as plasma proteins and hormones.
• Discontinuous Capillaries or Sinusoids have discontinuous endothelium with large gaps between the cells and a discontinuous basement membrane. Such capillaries are found in the liver and in blood-forming and lymphoid organs.

The layers/tunics of veins are not as well defined as in arteries. There are four types of veins:

• Venules (e.g. postcapillary venules, muscular venules). The venular endothelium has labile junctions that “open” in inflammatory reactions under the influence of histamine, serotonin, bradykinin, and other agents. The result is increased permeability and local swelling. Diapedesis is the exit of leukocytes from the vasculature, which occurs at postcapillary venules.
• Small veins are continuous with muscle venules, with a diameter less than 1mm.
• Small arteries may have up to 8 layers of smooth muscle in the tunica media, and may have an internal elastic membrane. Adventitia is very thin and inconspicuous.
• Medium veins are most of the named veins. Like small veins, they are characterized by a thin media containing only a few layers of smooth muscles. These vessels have a much thicker adventitia composed of collagen.
• Large veins are greater than 10mm (e.g. vena cavas, hepatic portal). All three tunics are present:
• Tunica Intima, which has an endothelial lining with its basal lamina, subendothelial connective tissue, and some smooth muscle cells. The boundary between intima and media is not clear.
• Tunica Media is relatively thin. Circumferentially arranged smooth muscle, collagen fibers, and fibroblasts.
• Tunica Adventitia is the thickest layer. It has longitudinally arranged smooth muscle cells.

Lymphatics are unidirectional and convey lymph. The smallest are called lymphatic capillaries, which are numerous in the loose connective tissue under the epithelium of the skin and mucous membranes. Capillaries coalesce to become lymphatic vessels.