Arteries carry blood from the heart to capillaries; smaller arteries are called arterioles. If we look at an artery in cross-section, we find three layers (or tunics) of tissues, each with different functions (Fig. 13–1).
The innermost layer, the tunica intima, is the only part of a vessel that is in contact with blood. It is made of simple squamous epithelium called endothelium. This lining is the same type of tissue that forms the endocardium, the lining of the chambers of the heart. As you might guess, its function is also the same: Its extreme smoothness prevents abnormal blood clot-ting. The endothelium of vessels, however, also pro-duces nitric oxide (NO), which is a vasodilator. The tunica media, or middle layer, is made of smoothmuscle and elastic connective tissue. Both of these tis-sues are involved in the maintenance of normal blood pressure, especially diastolic blood pressure when the heart is relaxed. The smooth muscle is the tissue affected by the vasodilator NO; relaxation of this mus-cle tissue brings about dilation of the vessel. Smooth muscle also has a nerve supply; sympathetic nerve impulses bring about vasoconstriction. Fibrous con-nective tissue forms the outer layer, the tunica externa. This tissue is very strong, which is important to prevent the rupture or bursting of the larger arter-ies that carry blood under high pressure.
The outer and middle layers of large arteries are quite thick. In the smallest arterioles, only individual smooth muscle cells encircle the tunica intima. As mentioned, the smooth muscle layer enables arteries to constrict or dilate. Such changes in diameter are regulated by the medulla and autonomic nervous sys-tem, and will be discussed in a later section on blood pressure.
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