How and why does gas exchange occur in tissues and lungs?

The gas exchange law states that individual gases flow from areas with higher partial pressures to areas with lower partial pressures, and this rule regulates the exchange of carbon dioxide and oxygen in tissues and lungs.

Normal alveolar PO2 is about 100 mmHg. The PO2 of venous blood that enters the lungs is 40 mm Hg. Therefore, oxygen moves down its partial (concentration) gradient from the alveoli to the capillaries. Cells continuously use oxygen for phosphorylation.