Assessment of acid–base balance

Acid–base balance is maintained by pulmonary elimination of carbon dioxide and renal excretion of non-volatile acids. An abnormal standard bicarbonate or base excess indicates a metabolic contribution to an acidosis or alkalosis, but does not indicate whether this is the primary disorder, part of a mixed disturbance, or related to normal physiological compensation. The respiratory response to metabolic acidosis or alkalosis is mediated by chemosensitive neurones in the brainstem medulla, which are stimulated under conditions of low pH within the cerebrospinal fluid (CSF). In respiratory disorders, derangement of Pco₂ is the primary abnormality. Metabolic acidosis is caused by addition of an acid other than CO₂ to body fluid, or by excessive loss of bicarbonate. Respiratory compensation for metabolic acidosis causes the arterial Pco₂ to fall approximately by 0.16 kPa for every mmol/L reduction in the serum HCO3-concentration. Respiratory compensation for metabolic acidosis begins within 30 minutes and is complete within 12-24 hours.