Why does the oxygen saturation in arterial blood sometimes fail to fully equilibrate with alveolar oxygen partial pressure during intense exercise, even when ventilation rate is high?

💡 Explanation

The arterial oxygen saturation may be limited by red blood cell transit time through the pulmonary capillaries. The diffusion of oxygen from alveoli into red blood cells requires a finite time; because during intense exercise the transit time decreases, full equilibration may not occur, therefore saturation decreases rather than increasing. This occurs despite high ventilation because diffusion capacity is limiting.

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