Abstract
This chapter describes the control of pressure and flow in the cardiovascular system. The control of the circulation may be viewed in two different ways: (1) control to ensure an adequate distribution of blood to the body as a whole and (2) control to adjust that distribution to provide for the differing demands of the different parts of the body. The control of arterial blood pressure depends on two factors: (1) the flow produced by the pump and (2) the resistance of the flow pathway. The blood pressure, therefore, is the product of the cardiac output and the resistance of the circulation to flow. Variations in flow to different organs are achieved by nervous stimuli controlling the constriction or dilatation of arterioles, arteriovenous shunts, and precapillary sphincters. There is some degree of intrinsic stabilization of blood flow to particular regions or organs despite changes in the perfusing pressure. This is termed autoregulation: it is mainly because of the ability of the walls of the blood vessels, like smooth muscle fibers elsewhere, to respond to stretching by contraction. Blood pressure and blood flow may also be modified by hormonal influences acting both in the body as a whole and more locally. A decrease in oxygen concentration or an increase in the other factors causes vasodilatation. Thus, the blood supply to an organ is usually governed by its metabolic need. The sympathetic system with its combination of activity through nervous pathways and the release of adrenaline from the adrenal medulla can control both the overall blood pressure and also flow to different organs.
Published Version
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