External and internal respiration. The lungs play a pivotal role in gas exchange between the air and the blood (external respiration). The oxygen (O2) is transported to the periphery, where the internal respiration takes place in the tissues. The pressure of oxygen varies from 160 mm Hg in the inhaled air to 100 mm Hg in the terminal air spaces (alveoli). Note also that the air is humidified, which lowers the pressure of oxygen from 160 to 150 mm Hg. Carbon dioxide (CO2) formed inside the tissues is carried by the blood from the tissues and exhaled in the lungs. PO2, partial pressure of oxygen; PCO2, partial pressure of carbon dioxide.