What are the expected PO2 levels in alveolar air compared to blood arriving at the alveoli?

The expected partial pressure of oxygen (PO2) levels in alveolar air is higher than in the blood arriving at the alveoli. In the alveoli, the average PO2 is around 104 mmHg, which reflects the oxygen content after gas exchange in the lungs has occurred. In contrast, the blood entering the alveoli has a lower PO2, typically around 40 mmHg, due to the fact that it has already delivered oxygen to the tissues throughout the body.

This difference in PO2 levels is essential for the process of diffusion, where oxygen moves from an area of higher concentration (the alveoli) to an area of lower concentration (the blood) to ensure that the blood is sufficiently oxygenated. The greater the difference in partial pressures, the more efficient the oxygen diffusion processes will be.

Thus, the correct answer illustrates the expected PO2 levels in the alveolar air compared to the blood arriving at the alveoli, highlighting the critical dynamics of gas exchange that takes place in the respiratory system.