Which statement best describes how the lungs act as an acid-base buffer?

• A. Increase respiratory rate and depth when CO2 levels in the blood are high, reducing acid load.
• B. Increase respiratory rate and depth when CO2 levels in the blood are low, reducing base load.
• C. Decrease respiratory rate and depth when CO2 levels in the blood are high, reducing acid load.
• D. Decrease respiratory rate and depth when CO2 levels in the blood are low, increasing acid load.

Answer: (A)

CO2 levels in the blood are tied to carbonic acid; the lungs help keep pH balanced by adjusting ventilation to change CO2, a rapid way to influence acidity. When CO2 is high, breathing faster and more deeply blows off CO2, which lowers the amount of carbonic acid and hydrogen ions in the blood. That reduces the acid load and nudges pH back toward normal. This is the action described in the best choice. If CO2 were low, increasing ventilation would push CO2 out further and worsen alkalosis, while slowing breathing when CO2 is high would let the acidity build up. The lungs thus act as a quick, breathing-based buffer to counteract pH shifts.