Final answer:
Blood entering the pulmonary capillaries has a pCO2 of about 45 mm Hg. The pCO2 of the alveoli is about 40 mm Hg, facilitating the diffusion of carbon dioxide from the blood to the lungs for exhalation.
Step-by-step explanation:
The pCO2 (partial pressure of carbon dioxide) of blood entering the arterial end of the pulmonary capillary bed is typically about 45 mm Hg. This measurement indicates that the blood coming from systemic circulation is rich in CO2. As the blood moves through the pulmonary capillaries, CO2 diffuses out of the blood and into the alveoli due to the pressure gradient where the alveolar pCO2 is around 40 mm Hg, lower than that of the blood. This pressure difference facilitates the transfer of carbon dioxide from the blood to the alveoli, where it can then be expelled from the body during the process of exhalation. The high solubility of CO2 compared to oxygen also ensures that despite the smaller pressure gradient, CO2 is effectively removed from the bloodstream.