Diffusion of gases occur in the alveolar region only and not in the other part of respiratory system, why ?
Alveoli have thin walls, have layer of mucus on inner side and have rich supply of blood. Each alveolus is made up of highly-permeable and thin layers. The barrier between the alveoli and the capillaries is thin and the diffusion of gases takes place from higher partial pressure to lower partial pressure. The pO2 of blood is lesser than alveolar air and pCO2 of blood is greater than alveolar air. Thus, alveoli support the diffusion of gase.
Since the other region do not provide the necessary conditions like thin and permeable membrane, difference in the partial pressure of the gases across the membrane etc., they do not supprt the diffusion of gases as alveoli.