Figure 1438

Physical model of the relationship between arterial pressure, arteriolar radius in different organs, and blood-flow distribution. In (a), blood flow is high through tube 2 and low through tube 3, whereas just the opposite is true for (b). This shift in blood flow was achieved by constricting tube 2 and dilating tube 3.

mean pressure—from about 90 mmHg to 35 mmHg— as blood flows through the arterioles (see Figure 14-34). Pulse pressure also diminishes to the point that flow beyond the arterioles—that is, through capillaries, venules, and veins—is much less pulsatile.

Like the model's outflow tubes (Figure 14-38), the arteriolar radii in individual organs are subject to independent adjustment. The blood flow through any organ is given by the following equation:

Forgan = (MAP — venous pressure)/Resistanceorgan

Since venous pressure is normally approximately zero, we may write:

Forgan = MAP/Resistanceorgan

Since the MAP, the driving force for flow through each organ, is identical throughout the body, differences in flows between organs depend entirely on the relative resistances offered by the arterioles of each organ. Ar-terioles contain smooth muscle, which can either relax and cause the vessel radius to increase (vasodilation) or contract and decrease the vessel radius (vasoconstriction). Thus the pattern of blood-flow distribution depends upon the degree of arteriolar smooth-muscle contraction within each organ and tissue. Look back at Figure 14-9, which illustrates the distribution of blood flows at rest; these are due to differing resistances in the various locations. Such distribution can be changed markedly, as during exercise, for example, by changing the various resistances.

How can resistance be changed? Arteriolar smooth muscle possesses a large degree of spontaneous activity (that is, contraction independent of any neural, hormonal, or paracrine input). This spontaneous contractile activity is called intrinsic tone (also termed basal tone). It sets a baseline level of contraction that can be increased or decreased by external signals, such as neurotransmitters. These signals act by inducing changes in the muscle cells's cytosolic calcium concentration (see Chapter 11 for a description of excitation-contraction coupling in smooth muscle). An increase in contractile force above the vessel's intrinsic tone causes vasoconstriction, whereas a decrease in contractile force causes vasodilation. The mechanisms controlling vasoconstriction and vasodilation in arte-rioles fall into two general categories: (1) local controls, and (2) extrinsic (or reflex) controls.

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Essentials of Human Physiology

Essentials of Human Physiology

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