SummaryThe author reviews our knowledge concerning the mechanism of CO2transfer in blood, its pulmonary clearance and the renal excretion of bicarbonate.He recalls the physiological process responsible for urine acidification and ammonia excretion.This knowledge is applied to the study of the disturbances of the acid- base equilibrium observed in clinical conditions. The use of the nomogram of Singer and Hastings is emphasized to recognise acidosis and alcalosis either of metabolic or respiratory origin.For example, the respiratory acidosis of emphysema, the metabolic acidosis of renal insufficiency and the hypokalemic alcalosis are studied in details.