For a gas with a high blood-gas partition coefficient, which factors influence transfer from alveoli to blood?

For a gas with a high blood-gas partition coefficient, which factors influence transfer from alveoli to blood?

• A. Gas is very soluble in blood; most is removed from the lung, and transfer is controlled only by blood flow
• B. Gas remains largely in alveoli; diffusion dominates
• C. Transfer is governed solely by alveolar surface area
• D. Gas is very soluble in blood; ventilation and diffusion govern absorption

Answer: (D)

When a gas dissolves readily in blood, the speed at which it moves from the alveoli into the blood is not limited mainly by diffusion across the membrane or by how much blood is flowing through the capillaries. Instead, the rate is set by how well you maintain the driving force (the alveolar partial pressure through ventilation) and by how quickly diffusion can occur across the alveolar-capillary barrier. In other words, both ventilation (to refresh alveolar gas and keep the pressure gradient) and diffusion (movement across the thin barrier) influence absorption when the gas is highly soluble in blood. This is why the statement that the transfer is governed by ventilation and diffusion is the best fit. The other ideas overemphasize one factor or misstate the limiting step: relying only on blood flow ignores the rapid equilibration that high solubility promotes; saying diffusion dominates ignores that ventilation also controls the driving pressure; and claiming it's solely about alveolar surface area ignores the role of the pressure gradient and membrane diffusion dynamics.

When a gas dissolves readily in blood, the speed at which it moves from the alveoli into the blood is not limited mainly by diffusion across the membrane or by how much blood is flowing through the capillaries. Instead, the rate is set by how well you maintain the driving force (the alveolar partial pressure through ventilation) and by how quickly diffusion can occur across the alveolar-capillary barrier. In other words, both ventilation (to refresh alveolar gas and keep the pressure gradient) and diffusion (movement across the thin barrier) influence absorption when the gas is highly soluble in blood.

This is why the statement that the transfer is governed by ventilation and diffusion is the best fit. The other ideas overemphasize one factor or misstate the limiting step: relying only on blood flow ignores the rapid equilibration that high solubility promotes; saying diffusion dominates ignores that ventilation also controls the driving pressure; and claiming it's solely about alveolar surface area ignores the role of the pressure gradient and membrane diffusion dynamics.