Oxygen Concentration in the Human Body

The PO2 decreases as distance away from the heart increases. When blood reaches the lungs through the pulmonary arteries, it has delivered oxygen to the body cells and picked up carbon dioxide, a waste product produced during respiration. Here, the partial pressure of oxygen is very low, typically 40 millimeters of mercury. This allows oxygen gas to naturally diffuse from the alveoli in the lungs into the capillaries of the circulatory system. The blood then leaves the lungs with a fresh supply of oxygen to begin its journey again. It is at this point, in the pulmonary veins that carry blood away from the lungs and back to the heart, that the partial pressure of oxygen is highest, typically 100 millimeters of mercury.