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