Which statement correctly assigns low and high blood-gas coefficients to the roles of gas in the lung?

Which statement correctly assigns low and high blood-gas coefficients to the roles of gas in the lung?

• A. Low bgc means gas stays in lungs; blood flow controls absorption; High bgc means gas dissolves in blood; ventilation and diffusion govern absorption
• B. Low bgc means the gas dissolves in blood; High bgc means the gas stays in lungs
• C. Low bgc means diffusion dominates; High bgc means blood flow dominates
• D. Low bgc implies gas remains in alveoli; perfusion determines transfer; High bgc implies dissolution in blood; diffusion and ventilation govern absorption

Answer: (A)

The key idea is how a gas’s solubility in blood (the blood–gas coefficient) shifts the bottleneck of transfer in the lungs. If a gas is poorly soluble in blood (low blood–gas coefficient), it takes longer to move from the alveolar air into the blood, so diffusion across the alveolar-capillary membrane is the rate-limiting step. In that case, the amount transferred is governed by diffusion. If a gas is highly soluble in blood (high blood–gas coefficient), it dissolves readily and equilibrates quickly with the blood. Here, the speed at which blood can deliver and carry away the dissolved gas becomes the limiting factor, so the process is perfusion-limited (controlled by blood flow). Thus, the correct pairing is: low blood–gas coefficient = diffusion dominates; high blood–gas coefficient = blood flow (perfusion) dominates. This excludes the idea that high solubility would be controlled mainly by ventilation or diffusion, which would contradict the perfusion-limited behavior of highly soluble gases.

The key idea is how a gas’s solubility in blood (the blood–gas coefficient) shifts the bottleneck of transfer in the lungs. If a gas is poorly soluble in blood (low blood–gas coefficient), it takes longer to move from the alveolar air into the blood, so diffusion across the alveolar-capillary membrane is the rate-limiting step. In that case, the amount transferred is governed by diffusion.

If a gas is highly soluble in blood (high blood–gas coefficient), it dissolves readily and equilibrates quickly with the blood. Here, the speed at which blood can deliver and carry away the dissolved gas becomes the limiting factor, so the process is perfusion-limited (controlled by blood flow).

Thus, the correct pairing is: low blood–gas coefficient = diffusion dominates; high blood–gas coefficient = blood flow (perfusion) dominates. This excludes the idea that high solubility would be controlled mainly by ventilation or diffusion, which would contradict the perfusion-limited behavior of highly soluble gases.