Final answer:
The PCO2 in alveolar capillaries decreases from 45 mm Hg to 40 mm Hg during external respiration, as carbon dioxide diffuses into the alveoli due to a pressure gradient and its higher solubility compared to oxygen.
Step-by-step explanation:
During external respiration, the partial pressure of carbon dioxide (PCO2) in alveolar capillaries decreases from 45 mm Hg to 40 mm Hg. This is due to a pressure gradient that drives carbon dioxide to diffuse down its pressure gradient, moving out of the capillaries and entering the alveoli where it can be exhaled. The solubility of carbon dioxide is much greater than that of oxygen, by a factor of about 20, which allows similar relative concentrations of these gases to diffuse across the respiratory membrane despite the smaller partial pressure difference for CO2 compared to oxygen.