Metabolic acidosis and alkalosis.

The factors controlling renal bicarbonate reabsorption and acid excretion under normal conditions and in the presence of metabolic acidosis and alkalosis are reviewed. The methods used to assess distal acidification and its limitations are also discussed. Measurement of urinary pCO2 in maximally alkaline urine (pH greater than 7.8) is a very useful qualitative method to assess distal acidification. The finding of a low urinary pCO2 in maximally alkaline urine indicates a distal acidification defect. We propose that both the secretory and gradient defect types of distal renal tubular acidosis are associated with a low urinary pCO2 when the urine is maximally alkaline. Sodium sulfate and neutral phosphate infusion may allow distinction between a secretory and gradient defect. Sodium sulfate lowers urine pH in the gradient defect but fails to produce the same response in the secretory defect. Neutral phosphate infusion when urine pH (6.8-7.4) is close to the pK of phosphate (6.8) results in an increase in urinary pCO2 in the gradient defect but not in the secretory defect. The mechanisms of generation, maintenance and treatment of metabolic alkalosis are also discussed.