Anatomy & Physiology

Anatomy & Physiology Study Guide

20.5 Veins Veins collect blood from all tissues and organs and return it to the heart. By definition, veins are blood vessels that return blood to the heart. The venous system has much lower pressure than the arterial system, and the walls of veins are much thinner than those of corresponding arteries. Veins are categorized by their size. Venules Venules are the smallest venous vessels and collect blood from the capillary beds. They vary widely in structure and size. An average venule has an inner diameter of roughly 20 µm. The smallest venules resemble expanded capillaries and lack a tunica media. Medium-Sized Veins Medium-sized veins range from 2-9mm in internal diameter, comparable in size tomuscular arteries. In these veins, the tunica media is thin, and the thickest layer is the tunica externa. These veins contain relatively few smooth muscle cells but have an abundance of longitudinal bundles of elastic and collagen fibers. Large Veins Large veins contain the superior and inferior venae cavae and their tributaries within the abdominopelvic and thoracic cavities. All large veins contain all the tunica layers. The slender tunica media is surrounded by a thick tunica externa composed of a mixture of elastic and collagen fibers. 20.6 Venous Valves The arterial system is a high-pressure system. Nearly all the force developed by the heart is required to push blood along the network of arteries and through the miles of capillaries. Blood pressure in a peripheral venule is only about 10 percent of that in the ascending aorta, and pressures continue to fall along the venous system. At times, valves do not work properly. When failure occurs, the most common cause is when the walls of the veins near the valves become weakened, stretched, or distorted. Blood then pools in the veins, and the vessels become grossly distended. The effects range from mild discomfort and a cosmetic problem, as in superficial varicose veins in the thighs and legs, to painful distortion of adjacent tissues, as in hemorrhoids. 20.7 The Distribution of Blood The total blood volume is unequally distributed among arteries, veins, and capillaries. The heart, arteries, and capillaries in the pulmonary and systemic circuits normally contain 30–35 percent of the blood volume (roughly 1.5 liters of whole blood), and the venous system contains the rest (65– 70 percent, or about 3.5 liters). Roughly one-third of the blood in the venous system (about a liter) is circulating within the liver, bone marrow, and skin. Large volumes of blood are housed in the extensive venous networks of these organs.

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