Acid-base homeostasis · USMLE Step 1

Which buffer system acts as the principal extracellular (plasma) buffer against acid–base disturbances in the body?

  1. Phosphate buffer system
  2. Plasma protein buffer system
  3. Bicarbonate–carbonic acid system
  4. Hemoglobin buffer system
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Correct answer: Bicarbonate–carbonic acid system

The bicarbonate–carbonic acid buffer system is the primary extracellular buffer in blood, efficiently resisting pH changes by shifting the equilibrium between CO₂, H₂CO₃ and HCO₃⁻.

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