Expansion
Bicarbonate reclamation proximally, then titratable acid and ammonium distally
Expansion
Two separate jobs:
1. Reclaim filtered bicarbonate (proximal tubule, about 80 to 90 per cent)
- Hydrogen ions are secreted, combine with filtered bicarbonate to form carbonic acid, which carbonic anhydrase converts to carbon dioxide and water
- These diffuse into the cell and reform bicarbonate, which leaves basolaterally
- No net acid excretion; this merely prevents bicarbonate loss
2. Generate new bicarbonate (distal nephron)
- Titratable acid: hydrogen ions buffered by filtered phosphate, accounting for about a third of net acid excretion
- Ammonium: glutamine is metabolised in the proximal tubule to ammonium and bicarbonate; the ammonium is excreted and the bicarbonate is new. This is the adaptable component, and can increase several-fold in chronic acidosis
Net acid excretion = titratable acid + ammonium - any bicarbonate lost.
Failure of step 1 gives proximal (type 2) renal tubular acidosis; failure of step 2 gives distal (type 1) renal tubular acidosis, in which urine cannot be acidified below pH 5.3.