Final answer:
The partial pressure of oxygen is lower in the alveoli compared to environmental air due to the diffusion of oxygen across tracheal membranes.
Step-by-step explanation:
The partial pressure of oxygen is lower in the alveoli than in environmental air as a result of diffusion of oxygen across tracheal membranes. When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. However, in cases when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without a large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane.