Exchange of gases in alveoli:

Alveoli are the primary site of exchange of gases. Concentration of oxygen is more in alveoli than in blood. So, oxygen diffuses into the blood, through, alveolar walls firstly, it passes into blood plasma and then combines with hemoglobin in RBCs and form oxy-hemoglobin. Co₂ has high concentration in lung capillaries, and Co₂ concentration in alveoli is less, so Co₂ diffuses into alveoli from capillaries. In this way exchange of gases occurs through alveolar walls.