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:

• Decreased oxygen

• Increased carbon dioxide

• Increased hydrogen ions

• Increased potassium ions

• Increased adenosine

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 return to the cells by way of the Na+-K+ pump. Finally, the release of adenosine is also believed to play a role in regulation of resistance vessels, particularly in the heart and skeletal muscle.

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.

Autoregulation. A different situation arises when the metabolic rate of a tissue remains constant, but the blood pressure changes. According to Ohm's law (Q = DP/R), an increase in blood pressure would tend to increase blood flow to a tissue. However, if the metabolic activity of the tissue does not change, then an increase in blood flow is unnecessary. In fact, blood flow to the tissue returns most of the way back to normal rather rapidly. The maintenance of a relatively constant blood flow to a tissue, in spite of changes in blood pressure, is referred to as autoregulation. Once again, resistance changes in the arterioles are involved.

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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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