How do the kidneys contribute to acid-base balance?

• A. By adjusting hydrogen ion secretion and bicarbonate reabsorption/secretion in nephron segments
• B. By filtering plasma proteins
• C. By filtering red blood cells
• D. By producing erythropoietin

Answer: (A)

Kidneys regulate acid-base balance mainly by adjusting hydrogen ion secretion and bicarbonate reabsorption and generation along the nephron. In the proximal tubule, most filtered bicarbonate is reabsorbed as carbon dioxide and water react to form bicarbonate again, a process driven by carbonic anhydrase. Hydrogen ions are secreted into the tubular lumen in exchange for sodium, and these secreted H+ ions combine with filtered bicarbonate to form carbonic acid, which dissociates to CO2 and water, effectively reclaiming bicarbonate into the blood. In the late distal tubule and collecting ducts, specialized cells actively secrete hydrogen ions into the urine via H+-ATPases, contributing to acid excretion. The secreted H+ can buffer with urinary phosphate and yield ammonium from glutamine metabolism, producing titratable acidity and ammonium excretion, which also helps generate new bicarbonate that enters the blood. This coordinated secretory and reabsorptive activity allows the kidneys to increase or decrease net acid excretion and regenerate bicarbonate in response to acid-base disturbances, maintaining blood pH. Other kidney roles, like filtering plasma proteins or red blood cells, or producing erythropoietin, are important for filtration and oxygen transport, but they do not directly regulate acid-base balance.