Exchange in Alveoli
The blood that enters the lungs from the heart has been routed through body tissue where mitochondria1 respiration has depleted its oxygen content. It thus has a low partial pressure of oxygen (Po2 = about 40 mm Hg). At the same time metabolic activity increases partial pressure of carbon dioxide in body tissue and partial pressure of carbon dioxide of blood entering the alveoli is 45 mm Hg. So compared to partial pressures in atmosphere blood entering lungs from the body tissue has low Po2 and high Pco2. The partial pressure of oxygen in alveoli is high and that of carbon dioxide is low so the gases diffuse only along their partial pressure gradient. The whole process of exchange is depicted in Figure.
Figure: Exchange of respiratory gases in lungs and tissue cells. Numbers represent partial pressures in milli meters of mercury (mm Hg).