Which statement best describes diffusion in the respiratory system?

Which statement best describes diffusion in the respiratory system?

• A. Oxygen diffuses from blood into alveoli due to higher PO2 in blood.
• B. Oxygen diffuses from alveoli into blood due to higher PO2 in alveoli.
• C. Diffusion is not involved; active transport drives gas exchange.
• D. Carbon dioxide diffuses from alveoli into blood because CO2 is more abundant there.

Answer: (B)

Gas exchange in the lungs happens by diffusion across the thin respiratory membrane, driven by differences in partial pressures. In the alveoli, the oxygen partial pressure is higher than in the blood arriving in the pulmonary capillaries, so oxygen moves down its gradient from alveoli into the blood. That’s why the statement describing oxygen diffusing from alveoli into blood because the alveolar PO2 is higher is the best description. The process is passive—not active transport—and carbon dioxide, which is more abundant in blood than in alveolar air, diffuses in the opposite direction (from blood into the alveoli) to be exhaled. The other options misstate the direction of the gradient or claim diffusion is not involved.

Gas exchange in the lungs happens by diffusion across the thin respiratory membrane, driven by differences in partial pressures. In the alveoli, the oxygen partial pressure is higher than in the blood arriving in the pulmonary capillaries, so oxygen moves down its gradient from alveoli into the blood. That’s why the statement describing oxygen diffusing from alveoli into blood because the alveolar PO2 is higher is the best description. The process is passive—not active transport—and carbon dioxide, which is more abundant in blood than in alveolar air, diffuses in the opposite direction (from blood into the alveoli) to be exhaled. The other options misstate the direction of the gradient or claim diffusion is not involved.