How does the renal system contribute to acid-base balance?

• A. Reabsorbs acids or excretes them in urine and produces bicarbonate.
• B. Produces carbon dioxide
• C. Converts bicarbonate to carbonate
• D. Regulates calcium balance only

Answer: (A)

The kidneys maintain acid-base balance by handling bicarbonate and hydrogen ions. They reclaim most filtered bicarbonate, excrete hydrogen ions into the urine, and generate new bicarbonate to replace what’s used to buffer acids. In the tubules, H+ is secreted into the lumen while bicarbonate is reabsorbed back into the blood, a process aided by carbonic anhydrase. The kidneys also generate new bicarbonate by metabolizing glutamine to ammonium, which is excreted as NH4+, with bicarbonate added to the blood. This combination keeps the blood buffer system ready to neutralize acids.

Other statements miss the mark: producing carbon dioxide is primarily a lung function; converting bicarbonate to carbonate isn’t the main renal mechanism for acid-base balance at physiological pH; and regulating calcium balance is a separate role.