Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 255 The cardiovascular system has five general classes of blood vessels. Arteries carry blood away from the heart. As they enter peripheral tissues, arteries branch repeatedly, and the branches decrease in diameter. Arterioles are the arterial branches with the smallest diameter. From the arterioles, blood moves into capillaries , where diffusion occurs between blood and interstitial fluid. From the capillaries, blood enters small venules , which unite to form larger veins that return blood to the heart. When blood exits the heart, it uses two passageways: the pulmonary trunk from the right ventricle, and the aorta from the left ventricle. The pulmonary arteries that branch from the pulmonary trunk carry blood to the lungs. The systemic arteries that branch from the aorta distribute blood to all other organs. Within these organs, further branching occurs, creating several hundred million tiny arterioles that provide blood to more than 10 billion capillaries. The most extensive branching occurs with the capillaries, which are barely the diameter of a single red blood cell. If every capillary in the human body were placed end to end, their combined length would exceed 25,000 miles, enough to circle the globe. Blood vessels must be resilient enough to withstand changes in pressure, and flexible enough to move with underlying tissues and organs. The pressures experienced by vessels vary with distance from the heart, and their structures reflect this fact. Moreover, arteries, veins, and capillaries differ in function, and these functional differences are associated with distinctive anatomical features. The Structure of V ssel Walls The walls of arteries and veins consist of three distinct layers—the tunica intima, tunica media, and tunica externa: • The tunica intima , or tunica interna, is the innermost layer of a blood vessel. This layer includes the endothelial lining and an underlying layer of connective tissue containing a variable number of elastic fibers. In arteries, the external margin of the tunica intima contains a thick layer of elastic fibers called the internal elastic membrane. • The tunica media , the middle layer, contains concentric sheets of smooth muscle tissue in a framework of loose connective tissue. The tunica media is bound to the tunica intima and tunica externa by collagen fibers. Commonly the densest layer in the wall of a small artery, a thin band of elastic fibers called the external elastic membrane, separate the tunica media (the thickest layer) from the surrounding tunica externa. The smooth muscle cells of the tunica media encircle the endothelium lining, the lumen of the blood vessel. When these smooth muscles contract, the vessel decreases in diameter; when they relax, the diameter increases. Large arteries also contain layers of longitudinally arranged smooth muscle cells. • The tunica externa , or tunica adventitia, is a connective tissue sheath and the outermost layer. In arteries, this layer contains collagen fibers with scattered bands of elastic fibers. In veins, elastic fiber networks and smoothmuscle cell bundles make the tunica externa of veins thicker of 368 Chapter 20: Blood Vessels and Circulation 20.1 Blood Vessels