Optimal cellular function occurs when the pH of the extracellular fluid is 7.4, which in terms of hydrogen ion concentration ([H+]) is 40 nanomoles/liter. There are, however, many instances, especially in disease, when the pH of plasma varies from the norm. Failure to maintain pH within the normal range changes the ionization status of chemical groups, altering the activity of enzymes and the integrity of cell membranes. Such pH changes are potentially life threatening and therefore require immediate correction. A state of abnormally low extracellular pH is referred to as acidosis and the opposite situation, a rise in pH, as alkalosis. Both reflect a failure in the regulation of extracellular [H+].

Normal metabolic activity continuously generates an acid load which is regulated within the body by:

1. – Intracellular and extracellular chemical buffers.

2. – Respiratory adjustment of CO2 concentration.

3. – Excretion of acid and the regeneration of body buffer systems by the kidney.

These regulatory mechanisms are complementary and the renal and respiratory systems have considerable compensatory capacity, the former by modulating the excretion of acid or base and the latter by varying the rate of CO2 venting.

Acid–base disturbances may be categorized broadly into those of either respiratory or metabolic origin. Whereas respiratory acidosis or alkalosis derives from primary respiratory dysfunction, metabolic acidosis or alkalosis may be of renal and alimentary origin. Metabolic acidosis may also arise from the generation of acid, particularly lactic acid, following severe or unaccustomed exercise.