Generation is one thing; maintenance requires impaired bicarbonate excretion
Expansion
The healthy kidney excretes excess bicarbonate readily, so an alkalosis can only persist if something prevents that. Maintenance factors:
- Volume depletion, which forces avid proximal sodium and bicarbonate reabsorption
- Chloride depletion, since bicarbonate reabsorption rises when chloride is scarce
- Hypokalaemia, which drives hydrogen ion secretion
- Mineralocorticoid excess
Chloride responsive (urine chloride under 20 mmol/l): vomiting, nasogastric aspiration, diuretics after they have worn off, post-hypercapnia. Treated with sodium chloride and potassium replacement.
Chloride resistant (urine chloride above 20): hyperaldosteronism, Cushing syndrome, Bartter and Gitelman syndromes, current diuretic use, severe potassium depletion. Saline does not correct these; the underlying cause must be treated.
Paradoxical aciduria in prolonged vomiting illustrates the conflict of priorities: despite alkalaemia, the volume-depleted kidney continues to reabsorb sodium and secrete hydrogen ions, producing an acid urine. Volume replacement is what allows the alkalosis to correct.