In the previous chapter, the contributions of various cardiovascular parameters, including peripheral vascular resistance, venous tone, and contractile state, to the regulation of cardiac output were examined. In this chapter, we explore the mechanisms that are directed toward regulating arterial blood pressure. The overall strategy of the cardiovascular system is to provide all organs with a constant perfusion pressure and to allow each individual organ to regulate its blood flow in accordance with the local needs of the tissue. In Chapter 11, it was noted that virtually all capillary beds perfusing the various tissues of the body are arranged in parallel and, thus, the driving force for blood flow, the mean arterial pressure (MAP), is the same for each tissue. Increased demand for blood flow within a particular organ activates local mechanisms that result in a reduction in arteriolar resistance. As long as MAP remains constant, blood flow into that tissue will increase as mandated by Ohm's law (see Chapter 11). MAP, however, remains constant only if cardiac output exhibits an equal increment. If cardiac output does not increase accordingly, a reduction in the vascular resistance of one organ would decrease total peripheral resistance, which in turn would decrease MAP and cause blood flow to fall in all of the other organs. The overall effect would be to divert a greater fraction of cardiac output through one organ at the expense of others. Hence, to prevent changes in blood flow to one organ from interfering with flow to the other organs, MAP must be invariant.
Cardiovascular control is designed so that MAP is closely monitored and normally held at approximately 100 mm Hg. If, for example, a decrease in MAP is sensed, the control systems will alter cardiac function and vascular resistance so that MAP is quickly returned to its normal value. To meet the moment-to-moment changes in local tissue metabolism, regional vascular resistances are continuously being adjusted, usually by local mechanisms that do not involve the central nervous system. Central control mechanisms, on the other hand, are directed toward sensing any change in MAP and reacting to such changes by adjusting the activities of the heart and vascular smooth muscle, thereby promptly restoring MAP to its normal value. The regulation of MAP involves both neural reflexes, which can restore MAP within seconds, and renal-hormonal pathways that have time constants measured in hours or even days. In the following sections, each component of blood pressure control is examined and its relative importance explained.
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