Which organ maintains long-term acid-base balance by reabsorbing bicarbonate and secreting hydrogen ions?

• A. Kidneys (renal tubules)
• B. Lungs
• C. Liver
• D. Spleen

Answer: (A)

Long-term acid-base balance is managed by the kidneys through their ability to reabsorb bicarbonate and to secrete hydrogen ions. This dual action adjusts the body's bicarbonate pool and removes acid, which slowly shifts blood pH toward a stable level over hours to days. In the kidneys, bicarbonate reabsorption mainly happens in the proximal tubule, while hydrogen ions are secreted into the tubular fluid in the distal tubule and collecting ducts, enabling the body to excrete acid and regenerate bicarbonate for ongoing buffering.

The lungs play a crucial role too, but in a different way: they quickly adjust pH by changing CO2 levels through breathing, providing rapid, short-term compensation rather than setting the long-term balance. The liver and spleen aren’t responsible for maintaining acid-base balance in this primary sense.

So, the organ that maintains long-term acid-base balance by reabsorbing bicarbonate and secreting hydrogen ions is the kidneys.