Which partial pressure is greater in diffusion limited gas exchange?

• A. Pa
• B. Pb
• C. Pc
• D. PA

Answer: (D)

In diffusion-limited gas exchange, the partial pressure of the gas in the alveoli (denoted as PA) is greater than the partial pressure of the gas in the blood (denoted as P). This situation arises because, during the completed phase of gas exchange, the gas does not fully equilibrate between the alveolar air and the blood in the pulmonary capillaries.

In a diffusion-limited scenario, the rate of gas transfer to the blood is limited by the diffusion process itself rather than by blood flow. As a result, when a gas such as carbon monoxide (CO) or another diffusion-limited gas is involved, the alveolar partial pressure will remain higher compared to the partial pressure in the pulmonary capillaries until the gas has sufficient time and conditions to diffuse across the alveolar-capillary membrane.

Understanding this concept is crucial for different pathological conditions where gas exchange can be impaired, leading to a significant difference between alveolar and arterial gas pressures. The partial pressure in the alveoli must be sufficiently high to drive diffusion into the blood, underlining the role of PA as the higher partial pressure in this context.