The blood vessels include the arteries, arterioles, veins, venules, and capillaries. Arteries are the vessels that carry blood from the heart; veins are vessels that carry blood to or toward the heart. Both types of vessels are composed of three coats in differing proportions : tunica intima, tunica media, and tunica externa.

The tunica intima, the smooth inner lining, runs uniterrupted throughout the vascular system and thus forms the lining of all the vessels; variable amounts of elastic tissue and smooth muscle occur in the other two coats. The larger blood vessels posses tiny vessels (called vasa vasorum) within their walls for nourishment.

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The walls of veins are quite a bit thinner than those of arteries. Unlike arteries, veins are collapsed when empty, and those of the lower extremities are equipped with valves to prevent backflow. As suggested by their names, the arterioles and venules are the smallest arteries and veins, the former leading blood into the capillaries and the later leading it from the capillaries. And, most important, the arterioles have relatively large amounts of smooth muscle, a fact accounting for their pronounced ability to alter their caliber when the occasion demands.

The capillaries are an interlacing network connecting the arterioles and venules. In a very real sense, these microscopic vessels are the heart of the cardiovascular system, for through their one celled walls nutrients and wastes are exchanged between the blood and tissue fluid. The capillaries are the ultimate element, as it were, in the maintenance of fluid balancd, because the volume of fluid (water, for the most part) leaving the capillaries is rather evenly balance by the fluid entering them. This accounts in large part for the constant volumes of blood and tissue fluid.