Where is the PO2 the highest in the body?

The partial pressure of oxygen (PO2) is highest in the alveoli compared to other locations in the body. This is because the alveoli are the sites of gas exchange in the lungs where oxygen enters the bloodstream. During inhalation, fresh air rich in oxygen is brought into the alveoli, and the PO2 in these air sacs reaches its peak level as oxygen diffuses from the alveoli into the pulmonary capillaries.

In contrast, the PO2 in arteries is lower than in the alveoli because, although arterial blood is oxygenated, it has already mixed with the relatively lower PO2 levels from venous blood returning to the heart. Veins have an even lower PO2, as they primarily carry deoxygenated blood back to the heart from the tissues. Additionally, the PO2 in tissues is the lowest, as oxygen is utilized by cells for metabolic processes. Therefore, the highest PO2 is clearly found in the alveoli during the process of respiration.