pH regulation (biology)
The processes operating in living organisms to preserve a viable acid-base state. 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, 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 solutions is pH, an expression representing the concentration (or preferably the activity) of hydrogen ions, [H+], in solution. The pH is defined as - log [H+], so that 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 Homeostasis
The most accessible and commonly studied body fluid is blood, and it, therefore, provides the most information on pH regulation. Blood pH in humans, and in mammals generally, is about 7.4. This value indicates that blood is slightly alkaline, because neutrality, the condition in which the concentration of hydrogen ions [H+] equals the concentration of hydroxyl ions [OH-], is pH 6.8 at mammalian body temperature of 98°F (37°C). The pH within cells, including the red blood cells, is typically lower by 0.2-0.6 unit, and is thus close to neutrality. In most animals other than warm-blooded mammals, blood pH deviates from the familiar value of 7.4. The major reason is that body temperature has an important influence on pH regulation. Consequently, animals that experience significant changes in body temperature have no single normal pH at which they regulate, but rather a series of values depending on body temperature.
Blood pH regulation is necessary because metabolic and ingestive processes add acidic or basic substances to the body and can displace pH from its proper value. For true regulation, active physiological mechanisms are required that can alter the acid-base composition of the blood in a controlled fashion. It is conventional to identify these control mechanisms with their effects on the principal buffer of the extracellular fluid, carbon dioxide (CO2). Carbon dioxide is produced by cellular metabolism and distributes readily throughout the body because of its high solubility and rapid diffusion. In solution, CO2 is hydrated to carbonic acid (H2CO3) which dissociates almost completely to H+ and bicarbonate ions [HCO3-]. Dissolved CO2 can be identified with a particular partial pressure of CO2 (PCO2). To regulate blood pH, organisms have mechanisms to independently control PCO2 and [HCO3-].
The cells, while benefitting from the stability afforded by whole body mechanisms (respiratory and ionic), also have local means for their own pH regulations. An acute acid load on a cell, whether from its intrinsic metabolism or from an external source, is dealt with first by the cell's chemical buffering capacity, a capacity that exceeds that of the blood by severalfold. Other cellular mechanisms include the conversion of organic acids to neutral compounds through metabolic transformations, and the transfer of acid equivalents from the primary cell fluid, the cytoplasm, into cellular organelles. See Biopotentials and ionic currents, Chemiosmosis, Enzyme