Acid-base homeostasis · USMLE Step 1

In chronic respiratory acidosis, what renal adaptation helps mitigate the acid load over days?

  1. Decreased HCO₃⁻ reabsorption in tubules
  2. Increased HCO₃⁻ generation and reabsorption
  3. Reduced ammonium excretion in urine
  4. Increased phosphate reabsorption
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Correct answer: Increased HCO₃⁻ generation and reabsorption

In chronic respiratory acidosis (elevated CO₂), kidneys increase generation and reabsorption of bicarbonate and excrete more H⁺ (e.g., via ammonium), helping restore pH toward normal.

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