The partial pressure of oxygen in the lungs is
100 mm Hg
The partial pressure of oxygen in the lungs or alveolar air is approximately 100 mm Hg, while that of oxygen in the venous blood in pulmonary arterial capillaries is 40 mm Hg on an average. Due to a PO2 difference between air and blood, oxygen diffuses rapidly from the alveolar air into the blood.