Which substance is used by the proximal tubule to buffer acid and regenerate bicarbonate?

• A. Bicarbonate
• B. Ammonia (NH3)
• C. Phosphate
• D. Ammonium (NH4+)

Answer: (B)

In the proximal tubule, acid handling and bicarbonate regeneration mainly rely on ammoniagenesis. The tubule cells generate ammonia (NH3) from glutamine. This NH3 diffuses into the tubular lumen and grabs a hydrogen ion (H+) that’s being secreted there, forming ammonium (NH4+). This trapping of H+ in the urine lets continued acid excretion occur without dropping urine pH, and the metabolism of glutamine also yields new bicarbonate that is reabsorbed into the blood, effectively regenerating bicarbonate. So NH3 is the buffering agent produced by the proximal tubule that enables acid buffering and bicarbonate regeneration. NH4+ is the excreted form after NH3 picks up a proton, not the buffering molecule itself.