Anatomy & Physiology

Anatomy & Physiology Study Guide • Age-related changes in blood vessels may be associated with arteriosclerosis: the inelastic walls of arteries become less tolerant of sudden pressure increases, which can lead to an aneurysm, whose rupture may (depending on the vessel) cause a stroke, myocardial infarction, or massive blood loss;calcium salts can be deposited on weakened vascular walls, increasing the risk of a stroke or myocardial infarction; and thrombi can form at atherosclerotic plaques. 20.17 Chapter Twenty Review Arteries, arterioles, capillaries, venules, and veins differ in size, structure, and functional properties: • Blood travels through a network of arteries, veins, and capillaries. All chemical and gaseous exchange between blood and interstitial fluid occurs across capillary walls. • Arteries and veins form an internal distribution system through which the heart propels blood. Arteries branch regularly, decreasing in size until they become arterioles. From the arterioles, blood goes into capillary networks. Blood flowing from the capillaries goes into small venules before entering larger veins. • The walls of arteries and veins have three layers: the innermost tunica intima, the tunica media, and the outermost tunica externa. • In general, the walls of arteries are thicker than those of veins. Arteries constrict when blood pressure does not distend them; veins constrict very little. The endothelial lining cannot contract, so when constriction occurs, the lining of an artery is thrown into folds. • The arterial system includes the large elastic arteries, medium-sized muscular arteries, and smaller arterioles. As we move toward the capillaries, the quantity of vessels rises, but the diameters of the individual vessels lower, and the walls become thinner. • Atherosclerosis, a kind of arteriosclerosis, is linked with changes in the endothelial lining of arteries. Fatty masses of tissue, known as plaques, typically develop during atherosclerosis. • Capillaries are the only blood vessels with walls thin enough to allow an exchange between blood and interstitial fluid. Capillaries are continuous or fenestrated. Sinusoids have fenestrated walls and form elaborate networks that allow very slow blood flow. Sinusoids are located in the liver and in various endocrine organs. • Capillaries form interconnected networks called capillary beds (capillary plexuses). A precapillary sphincter, which is a band of smooth muscle, adjusts the blood flow into each capillary. Blood flow in a capillary will change as vasomotion happens. The whole capillary bed may be bypassed by blood flow through arteriovenous anastomoses. • Venules get blood from the capillaries and merge into medium-sized veins and then large veins. The arterial system is high-pressure; blood pressure in veins is lower. Valves in veins stop the backflow of blood. • Peripheral venoconstriction maintains adequate blood volume in the arterial system after a hemorrhage. The venous reserve normally accounts for 20 percent of total blood volume.

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