Long-term maintenance of normal serum pH depends on the renal excretion of the daily acid load. Which organ is primarily responsible?

• A. Liver
• B. Kidneys
• C. Lungs
• D. GI tract

Answer: (B)

The body maintains normal serum pH over the long term mainly through the kidneys' ability to excrete daily nonvolatile acid load and regenerate bicarbonate. The kidneys accomplish this by secreting hydrogen ions and reabsorbing filtered bicarbonate in the proximal tubule, while also generating new bicarbonate through metabolism of glutamine and excreting acid as ammonium and titratable acids. This slow, ongoing process replenishes the bicarbonate buffer and removes acid produced by metabolism, keeping pH stable over days to weeks. The lungs influence pH by rapidly adjusting CO2 (volatile acid) via ventilation, which is a quick corrective mechanism but not the long-term solution. The liver and GI tract play roles in metabolism and minor bicarbonate handling, but they are not the primary systems for long-term acid-excretion balance. Therefore, the kidneys are responsible.