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The blood vessels consist of arteries, arterioles, capillaries, venules, and veins. All blood is carried in these vessels. The arteries, which are strong, flexible, and resilient, carry blood away from the heart and bear the highest blood pressures. Because arteries are elastic, they narrow (recoil) passively when the heart is relaxing between beats and thus help maintain blood pressure. The arteries branch into smaller and smaller vessels, eventually becoming very small vessels called arterioles. Arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease blood flow to a particular part of the body.
Capillaries are tiny, extremely thin-walled vessels that act as a bridge between arteries (which carry blood away from the heart) and veins (which carry blood back to the heart). The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into tissues and allow waste products to pass from tissues into the blood.
Blood flows from the capillaries into very small veins called venules, then into the veins that lead back to the heart. Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen (dilate) as the amount of fluid in them increases. Some veins, particularly veins in the legs, have valves in them, to prevent blood from flowing backward. When these valves leak, the backflow of blood can cause the veins to stretch and become elongated and convoluted (tortuous). Stretched, tortuous veins near the body's surface are called varicose veins (see Varicose Veins).
Blood Vessels: Circulating the Blood
If a blood vessel breaks, tears, or is cut, blood leaks out, causing bleeding. Blood may flow out of the body, as external bleeding, or it may flow into the spaces around organs or directly into organs, as internal bleeding.
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