Respiratory Mechanics and Gas Exchange · USMLE Step 1

A region of the lung receives ventilation but little blood flow. Which describes this situation and its effect on gas exchange?

  1. Shunt; CO₂ builds up and O₂ exchange is enhanced
  2. Dead space ventilation; O₂ and CO₂ exchange impaired
  3. Perfusion-limited exchange; gas exchange remains normal
  4. Diffusion-limited exchange; CO₂ removal increases
Show answer and explanation

Correct answer: Dead space ventilation; O₂ and CO₂ exchange impaired

Ventilation without adequate perfusion defines dead space ventilation, which reduces effective gas exchange (O₂ uptake and CO₂ removal) because blood flow is insufficient.

Difficulty: Medium Question 9 of 20

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