The thick muscle layer of arteries allows them to transport blood ejected from the heart under high pressure, and the elastic recoil of the large arteries further contributes to blood flow.The thinner muscle layer of veins allows them to distend when an increased amount of blood enters them, and their one-way valves ensure that blood flows back to the heart. Capillaries are composed of only one layer of endothelium, which facilitates the rapid exchange of materials between the blood and tissue fluid.
Blood vessels form a tubular network throughout the body that permits blood to flow from the heart to all the living cells of the body and then back to the heart. Blood leaving the heart passes through vessels of progressively smaller diameters, referred to as arteries, arterioles, and capillaries. Capillaries are microscopic vessels that join the arterial flow to the venous flow. Blood returning to the heart from the capillaries passes through vessels of progressively larger diameters, called venules and veins.
The walls of arteries and veins are composed of three coats, or "tunics." The outermost layer is the tunica externa, the middle layer is the tunica media, and the inner layer is the tunica interna. The tunica externa is composed of connective tissue, whereas the tunica media is composed primarily of smooth muscle. The tunica interna consists of three parts: (1) an innermost simple squamous epithelium, the endothelium, which lines the lumina of all blood vessels; (2) the basement membrane (a layer of glycoproteins) overlying some connective tissue fibers; and (3) a layer of elastic fibers, or elastin, forming an internal elastic lamina.
Although arteries and veins have the same basic structure (fig. 13.25), there are some significant differences between them. Arteries have more muscle for their diameters than do comparably sized veins. As a result, arteries appear more rounded in cross section, whereas veins are usually partially collapsed. In addition, many veins have valves, which are absent in arteries.
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