Respiratory Mechanics and Gas Exchange · USMLE Step 1

Which gas exchange scenario is considered perfusion-limited rather than diffusion-limited under normal conditions?

  1. O₂ transfer when alveolar-capillary membrane thickens
  2. N₂O uptake under normal membrane conditions
  3. CO₂ transfer in pulmonary edema
  4. O₂ transfer in interstitial fibrosis
Show answer and explanation

Correct answer: N₂O uptake under normal membrane conditions

Perfusion-limited gases (like N₂O) equilibrate rapidly across the membrane so the limiting factor is capillary blood flow — not the diffusion properties.

Difficulty: Medium Question 14 of 20

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