Vasomotion is an inherent contraction behavior of vascular smooth muscle that is not influenced by external input. The contractile state of precapillary vessels is also affected by changes in transmural pressure, which is the difference between intravascular pressure and extravascular pressure. If transmural pressure increases due to elevated intravascular pressure or arteriole dilation, the terminal arterioles contract. Conversely, a decrease in transmural pressure leads to relaxation of the precapillary vessels. Vasomotion can also be influenced by humoral and possibly neural factors. When increased transmural pressure causes contraction of the precapillary vessels, metabolic factors override the contractile response, leading to the abolition of vasomotion. This occurs when the oxygen supply becomes insufficient for the tissue's needs, such as during exercise in skeletal muscle.

Although a reduction in transmural pressure causes relaxation of the terminal arterioles, capillary blood flow cannot increase if the reduction in intravascular pressure is due to severe constriction of the upstream microvessels. Large arterioles and metarterioles also exhibit vasomotion, but their contraction usually does not completely block the vessel lumen and interrupt blood flow. In contrast, contraction of the terminal arterioles may halt blood flow. Consequently, the flow rate in capillaries can be altered by the contraction and relaxation of small arteries, arterioles, and metarterioles.

Capillary blood flow is known as nutritional flow since it facilitates the exchange of gases and solutes between the blood and the tissues. Conversely, blood flow that bypasses the capillaries by passing through metarterioles from the arterial to the venous side of circulation is termed nonnutritional or shunt flow. Some areas of the body, such as fingertips and ears, possess true AV shunts. However, in many tissues like muscle, anatomical shunts are absent. Nonetheless, nonnutritional flow can still occur even in the absence of these shunts. In tissues with metarterioles, nonnutritional flow may persist from arteriole to venule continuously during periods of low metabolic activity when many precapillary vessels are closed. As metabolic activity increases, more precapillary vessels open to allow capillary perfusion.

True capillaries lack smooth muscle and therefore cannot actively constrict. However, the endothelial cells forming the capillary wall contain actin and myosin, enabling them to change shape in response to specific chemical stimuli.

The narrow lumen of a thin-walled capillary allows it to withstand high internal pressures without bursting. This property can be explained using Laplace's law, specifically in the case of very thin-walled vessels like capillaries. Wall tension opposes the distending force (ΔPr) that seeks to pull apart a theoretical longitudinal slit in the vessel.
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 (