Oxygen diffuses from the alveolar air into the blood of the capillaries because
- A. the blood actively pumps it in
- B. the alveolar walls are impermeable to nitrogen
- C. the partial pressure of oxygen is higher in the alveoli than in the blood
- D. plasma proteins pull it across
Explanation
Gases always diffuse down their partial pressure gradients, and incoming alveolar air carries more oxygen than the arriving deoxygenated blood. At the tissues the gradient reverses, so oxygen leaves the blood there.
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