Acid-Base Equilibrium

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acid-base equilibrium

[′as·əd ′bās ‚ik·wə′lib·rē·əm]
The condition when acidic and basic ions in a solution exactly neutralize each other; that is, the pH is 7.

Acid-Base Equilibrium


the set of physicochemical and physiological processes making for the relative constancy of the pH of the internal medium of the organism.

The normal pH of human blood is kept to 7.35–7.47, despite the influx of acidic and basic products of metabolism. Constancy of the pH of the internal medium is a necessary condition for the normal occurrence of the life processes. Values of blood pH that go beyond the limits indicated are evidence of major disturbances in the organism. Values below 6.8 and above 7.8 are incompatible with life.

Numerous physiological systems participate in the maintenance of constant blood pH. In addition to these there are buffer systems, which consist of weak acids and their strong-base salts (for example, hemoglobin, which possesses the properties of a weak acid, and its salt, potassium carbonate; and carbonic acid, H2CO3, and its salt, sodium bicarbonate, NaHCO3).

The buffer-system mechanism of maintaining acid-base equilibrium can be demonstrated by the example of the cation of bicarbonate. If a strong acid, such as hydrochloric acid, enters the blood, it reacts with the bicarbonate to give the weak carbonic acid, which is almost without effect on the pH of the medium (NaHCO3 + HCL = NaCL + H2CO3). When a strong base enters the blood it reacts with the carbonic acid to give bicarbonate, which also has little effect on the pH. The capacity of the bicarbonate buffer should be rapidly exhausted by an accumulation of either carbonic acid or bicarbonates; this does not occur, however, thanks to the action of certain physiological systems, such as respiration and excretion, which restore the capacity of the buffer. Thus, when carbonic acid (carbon dioxide) accumulates, it is removed through the lungs, and excess bicarbonate is eliminated through the kidneys. A shift in blood pH in the acid direction is called acidosis; a shift in the alkaline direction is called alkalosis. The pH of the majority of tissue fluids is maintained at 7.1–7.4.


Robinson, J. R. Osnovy reguliatsii kislotno-shchelochnogo ravnovesiia. Moscow, 1969. (Translated from English.)


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