Regulation of blood flow through tissues

Blood flow to most tissues in the body is determined by the metabolic needs of those tissues. Metabolically active tissues require enhanced delivery of oxygen and nutrients as well as enhanced removal of carbon dioxide and waste products. In general, as the metabolic activity of a tissue increases, its blood flow increases. An important feature of the circulatory system is that each tissue has the intrinsic ability to control its own local blood flow in proportion to its metabolic needs. 

Active hyperemia. The increase in blood flow caused by enhanced tissue activity is referred to as active hyperemia. Assuming a constant blood pressure, then according to Ohm s law (Q = AP/R), the increase in blood flow is the result of a decrease in local vascular resistance. Tissue metabolism causes several local chemical changes that can mediate this metabolic vasodilation. These include  

As metabolism increases, oxygen consumption and carbon dioxide production are enhanced. The concentration of hydrogen ions is also enhanced as more carbonic acid (formed from carbon dioxide) and lactic acid are produced by the working tissue. Furthermore, the concentration of potassium ions in the interstitial fluid is increased. The rate of potassium release from the cells due to repeated action potentials exceeds the rate of potassium 

Each of these chemical changes promotes vasodilation of arterioles. In addition, the increase in tissue temperature associated with increased metabolism further contributes to metabolic vasodilation. The resulting increase in local blood flow restores these substances to their resting values. More oxygen is delivered and excess carbon dioxide, hydrogen and potassium ions, and adenosine are removed. 

Arteriolar resistance changes that take place in order to maintain a constant blood flow are explained by the myogenic mechanism. According to this mechanism, vascular smooth muscle contracts in response to stretch. For example, consider a situation in which blood pressure is increased. The increase in pressure causes an initial increase in blood flow to the tissue. However, the increased blood flow is associated with increased stretch of the vessel wall, which leads to the opening of stretch-activated calcium channels in the vascular smooth muscle. The ensuing increase in intracellular calcium results in vasoconstriction and a decrease in blood flow to the tissue toward normal. 

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