Acid–base balance: maintenance of plasma pH

Homeostatic control of plasma pH (range 7.38–7.42) – defence of the alkaline environment in the face of massive daily acid production – is an essential requirement for life. This is achieved through three lines of defence: physico-chemical buffering, rapid respiratory changes in pCO2, and slow renal changes in H+ excretion and HCO3− reabsorption and production. Disturbances in acid–base balance are described according to the cause of a primary change in either pCO2 (respiratory acidosis, respiratory alkalosis) or plasma HCO3− concentration (metabolic acidosis, metabolic alkalosis). Buffering and respiratory changes minimize changes in pH; full compensation is effected through renal changes in reabsorption of filtered HCO3− and secretion of H+, leading to generation of HCO3− to replete buffer stores. Factors influencing HCO3− reabsorption (primarily proximal tubule) include amount filtered, extracellular fluid volume and arterial pCO2. Generation of HCO3− along the nephron is influenced by availability and pK of urinary buffers (e.g. acid phosphate, creatinine), renal tubular fluid pH and formation of ammonium salts (e.g. ammonium sulphate). Clinical conditions in which metabolic and respiratory changes in acid–base status occur are considered as are the compensatory mechanisms which limit changes in pH. Full correction of these disturbances requires removal of the primary disturbance.