What is the typical PCO2 level in blood arriving at the alveoli in comparison to the alveolar air?

In the context of gas exchange in the lungs, the typical PCO2 levels in blood returning to the alveoli reflect the physiological processes involved in respiration. Blood that reaches the alveoli has a higher partial pressure of carbon dioxide (PCO2) than the air within the alveoli. This is primarily due to the metabolic activity of tissues that produce carbon dioxide as a byproduct, resulting in a PCO2 of approximately 46 mmHg in the venous blood.

On the other hand, the PCO2 of the alveolar air is usually around 40 mmHg. This difference in carbon dioxide levels facilitates the diffusion of CO2 from blood into the alveolar air, where it can be expelled from the body during exhalation. This exchange is critical for maintaining acid-base balance and ensuring the efficient removal of carbon dioxide from the bloodstream.

Thus, the correct comparison of PCO2 levels is 46 mmHg in the blood compared to 40 mmHg in the alveolar air, highlighting the normal process of diffusion that occurs as carbon dioxide is eliminated from the body.