Final answer:
The partial pressure of CO2 in blood passing through pulmonary capillaries is generally about 45 mmHg, leading to its diffusion into the alveoli due to the pressure gradient and its higher solubility compared to oxygen. The correct option is C.
Step-by-step explanation:
The partial pressure of CO2 within blood passing through the metarterioles of pulmonary capillaries is generally about 45 mmHg. When the partial pressure of carbon dioxide is 45 mm Hg in the blood and 40 mm Hg in the alveoli, carbon dioxide diffuses out of the blood into the alveoli because of the pressure gradient.
This movement is facilitated by the fact that carbon dioxide has a much higher solubility than oxygen, allowing for effective diffusion despite the small pressure gradient. The carbon dioxide is then exhaled during the process of external respiration. The correct option is C.