Final answer:
CO2 diffuses from the venous blood into the alveoli due to a pressure gradient, aided by its high solubility.
Step-by-step explanation:
Because the partial pressure of carbon dioxide (PCO2) in venous blood is higher than that of alveolar air, CO2 normally diffuses from the blood into the alveoli. In the lungs, CO2 dissociates from hemoglobin and diffuses across the respiratory membrane due to the pressure gradient; blood PCO2 is approximately 45 mm Hg, and alveolar PCO2 is about 40 mm Hg. The diffusion of gases like oxygen (O2) and CO2 is driven by these gradients, where each gas diffuses down its own pressure gradient independently. The solubility of carbon dioxide is much higher than that of oxygen, which aids in its efficient transfer across the respiratory membrane even though the pressure difference is less than that of oxygen. This solubility, along with the existing pressure gradient, ensures that CO2 diffuses from areas of higher to lower partial pressure, which, in this case, means moving from the venous blood into the alveoli within the lungs.