Which statement best describes gas diffusion across the alveolar membrane?

Which statement best describes gas diffusion across the alveolar membrane?

• A. Oxygen diffuses from alveoli into blood due to partial pressure gradient.
• B. Oxygen diffuses from blood into alveoli with no gradient.
• C. CO2 diffuses from alveoli into blood due to partial pressure gradient.
• D. Blood flow reverses to maintain ventilation.

Answer: (A)

Gas diffusion across the alveolar membrane is driven by differences in partial pressures. Oxygen moves from the alveolar air, where its partial pressure is high, into the blood in the pulmonary capillaries, where it is lower, so it diffuses down its gradient. Carbon dioxide does the opposite: it diffuses from blood, where its partial pressure is higher, into the alveolus to be exhaled. The statement describing oxygen diffusion from alveoli into blood due to the partial pressure gradient captures this correctly. The other options misstate either the presence of a gradient, the direction of diffusion for CO2, or describe an unrealistic mechanism like reversing blood flow to maintain ventilation.

Gas diffusion across the alveolar membrane is driven by differences in partial pressures. Oxygen moves from the alveolar air, where its partial pressure is high, into the blood in the pulmonary capillaries, where it is lower, so it diffuses down its gradient. Carbon dioxide does the opposite: it diffuses from blood, where its partial pressure is higher, into the alveolus to be exhaled. The statement describing oxygen diffusion from alveoli into blood due to the partial pressure gradient captures this correctly. The other options misstate either the presence of a gradient, the direction of diffusion for CO2, or describe an unrealistic mechanism like reversing blood flow to maintain ventilation.