Where is the partial pressure of oxygen (PO2) the lowest in the body?

The partial pressure of oxygen (PO2) in the body is lowest in the veins. This is primarily due to the physiological processes of gas exchange that occur in the tissues. As oxygen is delivered from the blood to the tissues, it is utilized for cellular respiration, leading to a decrease in the amount of oxygen in the venous blood returning to the heart.

In the arteries, especially those directly exiting the lungs, the PO2 is relatively high due to the oxygenated blood coming from the alveoli where gas exchange takes place. In the capillaries, the PO2 is also still high when compared to venous blood, as this is where the blood is transitioning from oxygen-rich to oxygen-poor as it delivers oxygen to the tissues. In the alveoli, the PO2 is the highest in the respiratory cycle since fresh atmospheric air is continuously mixed with the remaining air in the lungs, facilitating high levels of oxygen available for diffusion into the blood.

Consequently, as blood progresses from arteries to capillaries and finally returns through veins, the progressive decrease in PO2 indicates the lowest levels are found in the veins after oxygen has been depleted by the tissues.