Vascular Resistance Is Affected by Intrinsic and Extrinsic Control Mechanisms
As described in Chapter 22, the blood flow through any organ or tissue is determined by the perfusion pressure (arterial pressure minus venous pressure) and by the resistance of the blood vessels of the organ (and by no other factors), as follows:
Blood flow = Perfusion pressure/Vascular resistance
All the organs of the systemic circulation are exposed to the same perfusion pressure.
Fherefore, differences in blood flow to the various organs result solely from their different vascular resistances. The vascular resistance of an organ is determined mainly by the diameter of its arterioles. Thus, arteriolar vasodilation and vasoconstriction are the mechanisms that increase or decrease the blood flow in one organ relative to another organ.In general, the factors that affect arteriolar resistance can be divided into intrinsic and extrinsic factors. Extrinsic control involves mechanisms that act from outside a tissue, through nerves or hormones, to alter arteriolar resistance. Intrinsic control is exerted by local mechanisms within a tissue. For example, as described in Chapter 23, histamine is released from mast cells of a tissue in response to injury or during an allergic reaction. Histamine acts locally on the arteriolar smooth muscle to relax it. Dilation of the arterioles decreases arteriolar resistance and therefore increases blood flow to the tissue. Histamine is an example of a paracrine: a substance released from one type of cell in a tissue that acts on another cell type in the same tissue to alter its function. A second example of intrinsic control is the arteriolar dilation and increased blood flow during exercise in skeletal muscle. This example illustrates the general phenomenon of metabolic control of blood flow: tissues tend to increase their blood flow whenever their metabolic rate increases.
Although the arterioles in all tissues are affected by both intrinsic and extrinsic mechanisms, intrinsic mechanisms predominate over extrinsic mechanisms in the control of arterioles in the coronary circulation, brain, and working skeletal muscle. By contrast, extrinsic mechanisms predominate over intrinsic mechanisms in the control of blood flow to the kidneys, splanchnic organs, and resting skeletal muscle. Skin is an example of a tissue in which both intrinsic and extrinsic control mechanisms have strong influences. In general, local (intrinsic) control dominates extrinsic control in the critical tissues: those that must have sufficient blood flow to meet their metabolic needs on a second-by-second basis for an animal to survive. Extrinsic control dominates intrinsic control in tissues that can withstand temporary reductions in blood flow (and metabolism) to make extra blood available for the critical tissues.