Exchange of gases : The gases are exchanged in alveoli of lungs and cells of the body. The exchange of gases in alveoli occurs between blood and alveolar air.

The alveolar air has pO₂ 104 mm and pO₂ of blood is 40 mm of Hg. Thus, oxygen from alveolar air diffuses into blood till pO₂ of the blood becomes 95 mm.

The pCO₂ of alveolar air is 40 mm and of blood is 45 mm. Thus, CO₂ from blood diffuses into alveolar air. 

Exchange of gases in cells : The blood which is in contact with cells have pO₂ 95 mm and pCO₂ 40 mm while cells of the tissues have pO₂ 40 mm and pCO₂ 45 mm. Thus, oxygen from blood diffuses into the cells and CO₂ from cells diffuses into blood till PCO₂ becomes 46 mm of Hg and PO₂ becomes 40 mm. Now the blood is carried by veins to heart, then to lungs by pulmonary arch. In lungs it is purified.

Diagrammatic representation of exchanges of gases at the alveolus and the body tissues with blood and transport of oxygen and carbon dioxide