In perfusion limited gas exchange, the partial pressure of gas is typically:

• A. Higher than in the alveoli
• B. Lower than in the alveoli
• C. Equal to that in the alveoli
• D. Variable and dependent on blood flow

Answer: (C)

In perfusion limited gas exchange, the partial pressure of the gas in the blood is typically equal to that in the alveoli. This occurs because the gas transfer from the alveoli into the blood does not depend on the diffusion of gas across the alveolar membrane being the limiting factor; rather, it is the blood flow that limits the amount of gas that can be picked up.

In this scenario, once blood reaches the alveoli, the gas readily diffuses down its concentration gradient until the partial pressure in the blood equilibrates with that in the alveoli. As a result, because of the rapid equilibration of the gas, the partial pressures become equal, which allows for efficient gas exchange as long as perfusion (blood flow) is intact.

Understanding that the key limiting factor is blood flow helps clarify why the other options describing the partial pressures as higher, lower, or variable do not apply in the context of perfusion limited scenarios. In essence, the equilibrium of partial pressures is critical to maintaining effective gas exchange under normal physiological conditions.