Blood Vessels Health Article

Exchange vessels

Exchange vessels include capillaries and postcapillary venules. The walls of capillaries are composed of only a tunica intima (a thin layer of endothelial cells). The average diameter of the lumen is just large enough to allow erythrocytes (red blood cells) to pass through in single file. Exchange vessels are the site where gases, nutrients, and wastes are exchanged between blood and surrounding tissues.

There are three major type of capillaries: continuous, fenestrated, and discontinuous. Continuous capillaries are the most abundant type in the human body and are found in skin, muscle, lungs, and the central nervous system. They have low permeability and therefore allow only limited passage of substances across the capillary wall. Fenestrated capillaries are much more permeable than continuous capillaries; their walls contain circular pores or fenestrae closed by a thin diaphragm. Discontinuous capillaries, also called sinusoids, have gaps between endothelial cells that are large enough to allow even erythrocytes to pass through the capillary wall. They are found in the liver, spleen, and bone marrow, as well as some endocrine glands.

The capillary bed is a network of capillaries that connect arterioles with venules; there are typically 10 to 100 capillaries per bed. Arterioles give rise to either capillaries or metarterioles, vessels that are wider than true capillaries and directly connect arterioles to venules. True capillaries branch off arterioles or metarterioles and are encircled at their origin by the precapillary sphincter, permitting the regulation of blood flow into the capillary. Arteriovenous (A-V) shunts are anastomoses that bypass the capillary bed completely; they are frequently seen in tissues that require increased blood flow.

Veins and venules

Veins are blood vessels that carry blood from the capillary beds to the heart. Capillaries give rise to venules (small veins that have walls composed of a thin layer of endothelial cells), which in turn converge to form veins. Blood from the head, neck, and arms is carried to the superior vena cava, while the inferior vena cava receives blood from the trunk and legs; these large veins empty into the right atrium of the heart. The veins carry blood that is oxygen-poor, with the exception of the pulmonary vein, which carries oxygenated blood from the lungs to the heart.

The walls of veins are thinner and the lumens larger than those of arteries. They can accommodate a large blood volume and may act as blood reservoirs, containing up to 70% of the body's total blood volume. Veins and venules are therefore called capacitance vessels. Most veins have a system of valves, paired folds of the tunica intima that prevent the backflow of blood.