pH regulation (biology)
Jackson, Donald C. Division of Biology and Medicine, Brown University, Providence, Rhode Island.
Last reviewed:June 2016
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The processes operating in living organisms to regulate the concentration of hydrogen ions and preserve a viable acid-base state. The pH value is an expression for the effective concentration of hydrogen ions in solution [lower pH values indicate more acidic conditions, whereas higher pH values indicate basic (alkaline) conditions (Fig. 1)]. The activity of these ions affects the equilibria and kinetics of a wide variety of chemical and biochemical reactions. With regard to biology, it is critical to maintain and regulate pH throughout the body within a narrow range because nearly every physiological process is pH-sensitive. In higher animals, much of the body substance (60–70%) consists of complex solutions of inorganic and organic solutes. For convenience, these body fluids can be subdivided into the cellular fluid (some two-thirds of the total) and the extracellular fluid. The latter includes blood plasma and interstitial fluid, which is the film of fluid that bathes all the cells of the body. For normal function, the distinctive compositions of these various fluids are maintained within narrow limits by a process called homeostasis. A crucial characteristic of these fluid solutions is pH, which represents the concentration (or preferably the activity) of hydrogen ions, [H+], in solution. The pH is defined as −log [H+]; in the usual physiological pH range of 7 to 8, [H+] is exceedingly low, between 10−7 and 10−8 M. Organisms use a variety of means to keep pH under careful control because even small deviations from normal pH can disrupt living processes. See also: Acid and base; Cell biology; Cytochemistry; Homeostasis; pH
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