The renal mechanism of HCO3 reabsorption in the proximal tubule involves carbonic anhydrase. Which of the following correctly describes the process?

• A HCO3 is directly reabsorbed by the apical NBC cotransporter from the tubular lumen
• B HCO3 crosses the apical membrane directly via H+/HCO3 antiporter
• C Intraluminal CA II generates H+ which acidifies urine directly
• D H+ is secreted into the lumen via NHE3, combines with filtered HCO3 to form H2CO3, which is split by brush-border CA IV, and the resulting CO2 diffuses into the cell ✓

Explanation

In the proximal tubule, Na-H exchanger 3 (NHE3) on the apical membrane secretes H+ into the lumen in exchange for Na+ reabsorption. Luminal H+ combines with filtered HCO3 to form carbonic acid (H2CO3), which is rapidly dehydrated by brush-border carbonic anhydrase IV (CA IV) to CO2 + H2O. CO2 diffuses into the tubular cell, where cytoplasmic CA II catalyses its rehydration to H2CO3, which dissociates to H+ (for recycling via NHE3) and HCO3- (which exits basolaterally via the NBC cotransporter).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.