Physiological pH

Acid–base homeostasis is that part of biologic homeostasis which is concerned with the proper balance between acids and bases in the extracellular fluids, and therefore determines the body's extracellular fluids' pH. Many of the body's extracellular components (including the proteins on the exterior surfaces of its cells) are very sensitive for their 3D configurations (or "tertiary structures") on the extracellular pH. Sringent mechanisms therefore exist to maintain the pH within very narrow limits. Outside the acceptable range of pH, proteins are denatured, causing enzymes and cellular transmembrane ion channels (among others) to malfunction. In the worst cases death may occur if the situation remains unremedied.

In various animals, including humans, acid–base homeostasis is maintained by means of three interconnected systems. The first are the various chemical buffers which minimize pH changes that would otherwise occur in their absence. They do not correct pH deviations, but only serve to limit the extent of the change that would otherwise occur. These buffers include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system. Physiological corrective measures make use of, primarily, the bicarbonate buffer system. This is because abnormalities in the carbonic acid (H₂CO₃, or dissolved carbon dioxide) concentration in the blood plasma can rapidly be corrected by variations in the rate and depth of breathing (i.e. by hyperventilation or hypoventilation). Abnormally low or high plasma bicarbonate (HCO−
3) ion concentrations can be corrected by the excretion of H⁺ or HCO−
3 ions in the urine, in instances where the plasma pH is abnormal. Plasma pH abnormalities are known as acidemia or alkalemia depending on which way the pH has deviated from normal.

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