Why does the partial pressure of oxygen (PO₂) in the alveoli not fully equilibrate with the PO₂ in the pulmonary capillaries during gas exchange?

💡 Explanation

Alveolar and capillary PO₂ do not equilibrate fully because red blood cells pass through capillaries too quickly for complete diffusion. The limited transit time means oxygen uptake is rate-limited by perfusion rather than diffusion. Therefore, equilibration is incomplete, rather than being caused by alveolar collapse or complete oxygen binding.

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