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. 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.