Vasomotion is an intrinsic contractile behavior of vascular smooth muscle and is independent of external input. Changes in transmural pressure (intravascular pressure minus extravascular pressure) also influence the contractile state of precapillary vessels. An increase in transmural pressure, caused either by an increase in Pv or by dilation of arterioles, results in contraction of the terminal arterioles. A decrease in transmural pressure causes precapillary vessel relaxation. Humoral and possibly neural factors also affect vasomotion. For example, when increased transmural pressure causes the precapillary vessels to contract, the contractile response can be overridden and vasomotion abolished. This effect is accomplished by metabolic (humoral) factors when the O2 supply becomes too low for the requirements of parenchymal tissue, as occurs in skeletal muscle during exercise.
Although a reduction in transmural pressure relaxes the terminal arterioles, blood flow through the capillaries cannot increase if the reduction in intravascular pressure is caused by severe constriction of the upstream microvessels. Large arterioles and metarterioles also exhibit vasomotion. However, their contraction usually does not completely occlude the lumen of the vessel and arrest blood flow, whereas contraction of the terminal arterioles may arrest blood flow. Thus the flow rate in capillaries may be altered by contraction and relaxation of small arteries, arterioles, and metarterioles.
Blood flow through the capillaries has been called nutritional flow because it provides for exchange of gases and solutes between blood and tissue. Conversely, blood flow that bypasses the capillaries as it passes from the arterial to the venous side of the circulation via metarterioles has been termed nonnutritional, or shunt, flow (see
Fig. 17.22 ). In some areas of the body (e.g., fingertips, ears), true AV shunts exist (see
Fig. 17.37 ). However, in many tissues, such as muscle, anatomical shunts are lacking. Even in the absence of these shunts, nonnutritional flow can occur. In tissues with metarterioles, nonnutritional flow may be continuous from arteriole to venule during low metabolic activity, when many precapillary vessels are closed. When metabolic activity increases in these tissues, more precapillary vessels open to allow capillary perfusion.
True capillaries lack smooth muscle and are therefore incapable of active constriction. Nevertheless, the endothelial cells that form the capillary wall contain actin and myosin, and they can alter their shape in response to certain chemical stimuli.
Because of its narrow lumen (i.e., small radius), a thin-walled capillary can withstand high internal pressures without bursting. This property can be explained in terms of the law of Pierre-Simon Laplace:
Equation 17.16
T = tension in the vessel wall r = radius of the vessel
Laplace’s equation applies to very thin-walled vessels, such as capillaries. Wall tension opposes the distending force (ΔPr) that tends to pull apart a theoretical longitudinal slit in the vessel (