Acid-base homeostasis · USMLE Step 1

Why is the bicarbonate buffer system considered “open” in human physiology?

  1. Because it neutralizes only fixed acids
  2. Because CO₂ is exhaled by the lungs, restoring buffer components
  3. Because bicarbonate is permanently lost from plasma
  4. Because it operates only within the kidneys
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Correct answer: Because CO₂ is exhaled by the lungs, restoring buffer components

It's called an open buffer because CO₂ (volatile acid) can be removed by the lungs, enabling the system to continuously restore buffer balance rather than reaching equilibrium like a closed system.

Difficulty: Medium Question 12 of 19

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