Diffusion of gases occurs in the alveolar region only and not in the other parts of the respiratory system. Why?

Each alveolus is composed of a thin, highly permeable layer of squamous cells. Blood capillaries are also composed of layers of squamous cells. The oxygen-rich air enters the human body through the nose and reaches the alveoli. The deoxygenated blood from the body is transported to the heart through a vein. The heart pumps it into the lungs to supply oxygen. The exchange of O₂ and CO₂ occurs between the capillaries surrounding the alveoli and the gas present in the alveoli. Therefore, the alveoli are the place for gas exchange. Due to pressure or concentration differences, gas exchange is carried out by simple diffusion. The barrier between the alveoli and, by extension, the capillaries is extremely thin, allowing gases to diffuse from higher partial pressures to lower partial pressures. The blood that reaches the alveoli has the lower partial pressure of O₂ and better partial pressure of CO₂ as compared to alveolar air. Hence, oxygen gas diffuses into the blood. Simultaneously, CO₂ diffuses out of the blood and into the alveoli.