Final answer:
Oxygen passes through the alveolar wall, interstitial space, and capillary wall, before binding to hemoglobin in the erythrocytes within the pulmonary capillaries.
Step-by-step explanation:
Gas Exchange in Alveoli
To get from an alveolus to the pulmonary capillaries, oxygen must pass through several structures. Firstly, the oxygen in the alveoli diffuses across the alveolar wall, which is a thin barrier made of alveolar epithelial cells. Next, it moves through the interstitial space, the small fluid-filled area between the alveolar wall and the capillary wall. Oxygen then traverses the capillary wall, a thin membrane of the capillary's endothelial cells. Upon crossing these barriers, oxygen binds with hemoglobin in the erythrocytes (red blood cells) inside the capillaries. Concomitantly, carbon dioxide travels in the opposite direction, from the capillary blood to the alveoli, following the same path but in reverse.