Which statement about oxygen diffusion during gas exchange is true?

Which statement about oxygen diffusion during gas exchange is true?

• A. Oxygen diffuses across the alveolar–capillary membrane into the blood.
• B. Oxygen diffuses from the atmosphere directly into red blood cells in the bloodstream.
• C. Oxygen diffusion occurs only in large airways.
• D. Oxygen diffusion occurs exclusively in systemic capillaries.

Answer: (A)

Oxygen diffusion during gas exchange is driven by diffusion across the thin alveolar–capillary membrane, from the air inside the alveoli into the blood in the pulmonary capillaries. The alveolar air has a high partial pressure of oxygen, while the blood entering the pulmonary capillaries has a lower partial pressure. Because the barrier between air and blood is extremely thin and presents a large surface area, oxygen readily moves down its partial pressure gradient into the blood. Once in the blood, oxygen binds to haemoglobin in red blood cells for transport to tissues. This is why the statement about oxygen crossing the alveolar–capillary membrane into the blood is correct. Oxygen does not diffuse directly into red blood cells from the atmosphere, and gas exchange mainly occurs in the alveoli, not in large airways, nor exclusively in systemic capillaries.

Oxygen diffusion during gas exchange is driven by diffusion across the thin alveolar–capillary membrane, from the air inside the alveoli into the blood in the pulmonary capillaries. The alveolar air has a high partial pressure of oxygen, while the blood entering the pulmonary capillaries has a lower partial pressure. Because the barrier between air and blood is extremely thin and presents a large surface area, oxygen readily moves down its partial pressure gradient into the blood. Once in the blood, oxygen binds to haemoglobin in red blood cells for transport to tissues.

This is why the statement about oxygen crossing the alveolar–capillary membrane into the blood is correct. Oxygen does not diffuse directly into red blood cells from the atmosphere, and gas exchange mainly occurs in the alveoli, not in large airways, nor exclusively in systemic capillaries.