Renal Tubular Physiology — Bicarbonate Handling

Total filtered (24×180) = 4320 mmol/day
Completely reabsorbed

PCT (85%) / TAL (10%)

• Absorbed as CO2 after conversion by C.A. 4 on brush border
• Converted back via CA2
• Reabsorbed to peritubular capilaries via Na/HCO3 cotransporter

DCT/CCD (5%)

• Type A intercalated cells
  • Secrete H via H-ATPase or H/K ATPase
    • ↑ by aldosterone
  • HCO3 is absorbed into peritubular capilary
• Type B intercalated cels
  • Secrete HCO3 via luminal Cl/HCO3 exchanger
    

pH changes

• PCT pH = 7.4.
• Reabsorption of Bicarb → pH ~ 6.8 at descending limb of LoH
• End of DCT → pH ~ 4.4

Hypoventilation:

• ↓RR → ↑PaCO ₂ → acidosis
• Chronic Acidosis (traditional)
  • → ↑ secretion of titratable acids (phosphate, Ammonia)
  • → Binding and secretion of additional H
  • → net ↑[HCO3] reabsorption in tubular cells (traditional model)
• Chronic Acidosis (stewart)
• ↑ ammonigenesis → ↑DCT ammonia assoc with ↑Cl secretion via luminal Cl/HCO3 exchanger → ↑SID → defense of acidosis.

Mooney 2016

Answer by Mooney