Why does oxygen move from the alveoli into the pulmonary capillary blood?
Quick answer: gases move spontaneously from an area of high partial pressure to one of lower pressure.
The partial pressure of
This difference in partial pressures of
The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick, so the exchange surfaces are very thin, and they are in close contact with each other.
Oxygen therefore diffuses quickly through the alveolar walls and into the capillaries.
Here is a great animation of gas exchange across the alveolar walls