Final answer:
The partial pressure of oxygen in the blood of the pulmonary capillaries is generally about 40 mmHg, which facilitates the diffusion of oxygen from the alveoli into the blood due to the pressure gradient.
Step-by-step explanation:
The partial pressure of oxygen within blood passing through the metarterioles of pulmonary capillaries is generally about 40 mmHg.
This is due to the fact that while the solubility of oxygen in blood is not very high, there is a significant difference in the partial pressure of oxygen between the alveoli and the blood in the pulmonary capillaries.
The strong pressure gradient, which is approximately 64 mm Hg (with alveoli having a partial pressure of about 104 mm Hg and blood in the capillaries at about 40 mm Hg), drives oxygen to rapidly diffuse from the alveoli into the blood.