Disorders of acid-base equilibrium are common in critically ill and injured patients. The presence of these disorders often signals severe underlying pathophysiology and, particularly in the case of metabolic acidosis, is a significant marker of adverse outcome. The traditional theory makes us believe that the hydrogen ion concentration (pH) in blood is mainly influenced by balance between the carbon dioxide and the bicarbonate ions in the blood. An acid load sufficient to reduce an unbuffered solution to a pH less than 2 only reduces the blood pH of an animal by 0.3 pH units. Disturbances of the acid-base equilibrium have their own morbidity and mortality. A blood pH less than normal (normal range 7.35-7.45) is called acidaemia; the underlying process causing acidaemia is called acidosis. Similarly, alkalaemia and alkalosis refer to a raised pH and the underlying process, respectively.