Arteries and Arterioles

Arteries are strong, elastic vessels that are adapted for carrying the blood away from the heart under high pressure. These vessels subdivide into progressively thinner tubes and eventually give rise to the finer branched arterioles (ar-te're-olz).

The wall of an artery consists of three distinct layers, or tunics, shown in figure 15.24. The innermost layer, tunica interna (intima), is composed of a layer of simple squamous epithelium, called endothelium, that rests on a connective tissue membrane that is rich in elastic and collagenous fibers.

The endothelial lining of an artery provides a smooth surface that allows blood cells and platelets to flow through without being damaged. Additionally, endothelium helps prevent blood clotting by secreting biochemicals that inhibit platelet aggregation (see chapter 14, p. 568). Endothelium also may help regulate local blood flow by secreting substances that either dilate or constrict blood vessels.

The middle layer, tunica media, makes up the bulk of the arterial wall. It includes smooth muscle fibers, which encircle the tube, and a thick layer of elastic connective tissue. The connective tissue gives the vessel a tough elasticity that enables it to withstand the force of blood pressure and, at the same time, to stretch and accommodate the sudden increase in blood volume that accompanies ventricular contraction.

The outer layer, tunica externa (adventitia), is thin and chiefly consists of connective tissue with irregular elastic and collagenous fibers. This layer attaches the artery to the surrounding tissues. It also contains minute vessels (vasa vasorum) that give rise to capillaries and provide blood to the more external cells of the artery wall.

The sympathetic branches of the autonomic nervous system innervate smooth muscle in artery and arte-riole walls. Vasomotor fibers stimulate the smooth muscle cells to contract, reducing the diameter of the vessel. This is called vasoconstriction (vas"o-kon-strik'-shun). If vasomotor impulses are inhibited, the muscle fibers relax and the diameter of the vessel increases. This is called vasodilation (vas"o-di-la'shun). Changes in the diameters of arteries and arterioles greatly influence blood flow and pressure.

Although the walls of the larger arterioles have three layers similar to those of arteries, the middle and outer layers thin as the arterioles approach the capillaries. The wall of a very small arteriole consists only of an endothelial lining and some smooth muscle fibers, surrounded by a small amount of connective tissue (figs. 15.25 and 15.26). Arterioles, which are microscopic continuations of arteries, give off branches called metar-terioles that, in turn, join capillaries.

The arteriole and metarteriole walls are adapted for vasoconstriction and vasodilation in that their muscle fibers respond to impulses from the autonomic nervous system by contracting or relaxing. Thus, these vessels help control the flow of blood into the capillaries.

Sometimes metarterioles connect directly to venules, and blood entering them can bypass the capillaries. These connections between arteriole and venous pathways, shown in figure 15.27, are called arteriovenous shunts.

99 Describe the wall of an artery.

^9 What is the function of the smooth muscle in the arterial wall?

^9 How is the structure of an arteriole different from that of an artery?

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