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Copyright © 2004, Elsevier Science (USA) All rights reserved.

Chapter 15 explained how the cardiovascular system has multiple control systems, all directed at maintaining a constant mean arterial pressure. An advantage of this effort to maintain a constant blood pressure is that each organ system is ensured a high and relatively stable pressure to drive blood flow through its vascular bed. Nonetheless, the burden of responsibility for regulating the rate of blood perfusion in an organ lies within that tissue. Placing the responsibility for blood flow control at the local level appears appropriate in view of the widely varying needs of different tissues for perfusion. In some tissues, such as skin and kidney, the blood flow at any given moment is related largely to the special functions of those organs. However, in most organs, the metabolic needs of the tissue dictate the level of blood perfusion, and several intrinsic control mechanisms exist to ensure that these needs are met by the vasculature. Figure 1 illustrates how cardiac output (total body blood flow) is distributed between tissues under resting conditions. It is readily apparent that blood flow, whether expressed as the total flow per organ or as a fraction of cardiac output, varies substantially between organs.

FIGURE 1 The distribution of cardiac output to different organs. Organ blood flows are shown as percentage of cardiac output and as absolute values for the organ or organ system.

This chapter explores the factors that enable the different organs in the body to regulate blood flow in accordance with their specific functional and metabolic needs. Special attention is given to those organ systems that exhibit unique blood flow requirements or that regulate blood flow through special mechanisms. (Note that local regulation of blood flow in the lung and kidney are discussed in Chapters 20 and 24, respectively.) The unique anatomic arrangement and blood flow distribution of the fetal circulation are also discussed.

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