Which type of gas exchange occurs when gas does not equilibrate by the time blood reaches the end of the capillary?

• A. Perfusion limited gas exchange
• B. Convection limited gas exchange
• C. Diffusion limited gas exchange
• D. Equilibration limited gas exchange

Answer: (C)

The type of gas exchange that occurs when gas does not equilibrate by the time blood reaches the end of the capillary is diffusion limited gas exchange. In this process, the rate of gas transfer across the alveolar-capillary membrane is primarily governed by the diffusion properties of the gas, rather than by the blood flow (perfusion).

When a gas is diffusion limited, it means that the gas’s partial pressure gradient remains significant throughout the entire length of the capillary, preventing the gas from reaching equilibrium with the blood by the time it exits the capillary. This scenario can occur with certain gases, such as carbon monoxide or during conditions of impaired diffusion, where the thickness of the membrane or surface area available for exchange might be affected.

In contrast, perfusion limited gas exchange occurs when blood flow is the limiting factor, and the gas equilibrates quickly; equilibration is reached before the blood exits the capillary. Convection limited gas exchange is not a standard term in the context of pulmonary physiology and typically refers to transport mechanisms in other contexts, such as in large airways or systemic circulation. Finally, equilibration limited gas exchange is also not a recognized category in respiratory physiology, as it does not align with the main principles governing gas exchange in