USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Which gas exchange process is affected by decreased compliance in the lungs?

1. Perfusion-limited gas exchange

2. Diffusion-limited gas exchange

3. Pathophysiologic mechanisms that reduce diffusing capacity

4. Surfactant deficiency

The correct answer is: Perfusion-limited gas exchange

Decreased lung compliance primarily affects tidal volume and the ability of the lungs to expand during inhalation. This condition is often seen in restrictive lung diseases, where the lungs cannot inflate adequately. In perfusion-limited gas exchange, the exchange of gases occurs quickly and is primarily dependent on blood flow (perfusion) rather than the ability of gases to diffuse across the alveolar membrane. When there is decreased compliance, the lungs cannot expand sufficiently, leading to reduced lung volumes. This decreased volume translates to smaller surface area available for gas exchange. While perfusion may remain the same, the reduced surface area limits the amount of gas that can be exchanged effectively, thereby impacting the overall efficiency of gas exchange. This is distinct from diffusion-limited gas exchange, where gas exchange is limited by the properties of the membrane or the partial pressures, rather than lung compliance. Because of these dynamics, the gas exchange process that is most notably affected by decreased lung compliance is perfusion-limited gas exchange.