Most organs and tissues manifest an increased blood flow (hyperemia) when their metabolic activity is increased; this is termed active hyperemia. For example, the blood flow to exercising skeletal muscle increases in direct proportion to the increased activity of the muscle. Active hyperemia is the direct result of arteriolar dilation in the more active organ or tissue. The factors acting upon arteriolar smooth muscle in active hyperemia to cause it to relax are local chemical changes in the extracellular fluid surrounding the arterioles. These result from the increased metabolic activity in the cells near the arterioles. The relative contributions of the different factors implicated vary, depending upon the organs involved and on the duration of the increased activity.
The most obvious change that occurs when tissues become more active is a decrease in the local concentration of oxygen, which is used in the production of ATP by oxidative phosphorylation. On the other hand, a number of other chemical factors increase when metabolism exceeds blood flow, such as carbon dioxide (an end product of oxidative metabolism), hydrogen ions (for example, from lactic acid), and potassium ions (accumulated from repeated action
potential repolarization). Local changes in these chemical factors have been shown to cause arteriolar dilation under controlled experimental conditions, and they all probably contribute to the active hyperemia response in one or more organs.