PHYSIOLOGY OF THE SYSTEMIC CIRCULATION



The aortic wall contains elastic fibers that allow it to expand with the expulsion of blood from the left ventricle, somewhat damping the pulse pres­sure generated and aiding diastolic flow to the coronary arteries with its recoil. The aorta suc­cessively branches into smaller and smaller ves­sels until arterioles, the major determinants of re­sistance in the systemic circulation, are reached (see Fig. 1—3). The arterioles contain a vascular sphincter that modulates blood flow dependent on regional metabolic needs; for example, aci­dosis and decreased oxygen tension increase re­gional perfusion^ and vice versa. The capillaries consist of a single endothelial cell layer and allow diffusion of oxygen, nutrients, C02, and waste products. The capillaries lead into the venous sys­tem, where blood is eventually delivered back to the right atrium. The flow of blood returning to the heart is aided by the valves in the venous sys­tem, which prevent reverse flow, particularly in the larger veins of the legs. The “milking” action of the muscles of the arms and legs and the pres­sure changes in the thoracic cavity also help to return blood to the heart. The veins have consid­erably thinner walls than the arteries and can ac­commodate a larger blood volume under low pres­sures (capacitance vessels). Vasoconstriction or vasodilation of the venous system can control the amount of blood returning to the heart. More of the total blood volume is located in the venous than in the arterial portion of the circulation. The lymphatic vessels also contribute to the return of fluid from the periphery.The major terminal ves­sel of the lymphatic system is the thoracic duct, which usually empties into the left brachioce­phalic vein.







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