External respiration or pulmonary gas exchange takes place in the lungs, between the external environment and the blood capillaries of the lungs. External respiration removes carbon dioxide from the blood and refills the blood with oxygen. The exchange of gas is taken due to a pressure gradient.
The gases diffuse from the air into the capillary blood of lungs owing to the difference in the partial pressures of the two gases. Exchange of carbon dioxide with oxygen cannot occur if the pressures of the gases are the same. The pressure of a gas is a function of the concentration of that gas in the mixture. The movement of gas occurs from its high concentration region to the area of its low concentration. As the oxygen concentration is higher in the air than in the blood, it diffuses into the blood.