A patient with chronic kidney disease has serum bicarbonate 14 mEq/L, pH 7.30, and pCO₂ 30 mmHg. Which buffering mechanism is most important for excreting the daily fixed acid load in CKD?

• A Extracellular bicarbonate buffering is the exclusive mechanism in CKD
• B Intracellular protein buffering replaces all other buffers
• C Renal bicarbonate regeneration via H⁺-ATPase in collecting duct increases to compensate fully
• D Bone carbonate buffering is the most significant chronic buffer; urinary phosphate and ammonia buffers are reduced in CKD ✓

Explanation

In advanced CKD, impaired ammoniagenesis (reduced glutaminase activity) and diminished phosphate availability reduce urinary acid excretion. Daily fixed acid (60-70 mEq) cannot be adequately excreted and is buffered chronically by skeletal bone mineral (carbonate and phosphate salts), causing demineralization and renal osteodystrophy. This is distinct from acute metabolic acidosis where extracellular bicarbonate and intracellular proteins are the immediate buffers. Treatment with oral bicarbonate supplementation can slow CKD progression by reducing bone buffering demand.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.