The amount of blood that flows through a capillary bed is controlled by the smooth muscle of the arteries and arterioles feeding that bed. Figure 49.18 illustrates the flow of blood in a typical capillary bed. Blood flows into the bed from an arteriole. Smooth muscle “cuffs,” or precapillary sphincters, on the arteriole can shut off the supply of blood to the capillary bed. When the precapillary sphincters are relaxed and the arteriole is open, blood flow through the capillaries increases.

Autoregulation depends on the sensitivity of the smooth muscle to its local chemical environment. Low O₂ concentrations and high CO₂ concentrations cause the smooth muscle to relax, thus increasing the supply of blood, which brings in more O₂ and carries away CO₂—a response known as hyperemia, which means “excess blood.” Increases in other by-products of metabolism, such as lactic acid, hydrogen ions, potassium, and adenosine (all of which increase in exercising muscle), also promote hyperemia. Hence activities that increase the metabolism of a tissue also induce hyperemia in that tissue.