Why does the pulmonary capillary endothelium, rather than the alveolar epithelium, primarily facilitate oxygen diffusion into the bloodstream during strenuous exercise?

💡 Explanation

The pulmonary capillary endothelium lies directly adjacent to the alveolar space, thus the oxygen has less distance to diffuse across, because Fick's Law of Diffusion dictates rate is inversely proportional to distance. Therefore, endothelial proximity enables faster oxygen exchange, rather than epithelial transport or hemoglobin affinity changes.

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