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

Explanation

A gas diffuses across a membrane from the side where its partial pressure is higher to the side where its partial pressure is lower. External respiration depends upon the principle of diffusion, as such for efficient gaseous exchange respiratory surface should have the following characteristics.
(1) It should have a large surface area.
(2) It should be extensive, thin and highly vascular.
(3) It must be permeable to respiratory gases-`O_2` and `CO_2`. (4) It must be moist at all times.
Alveoli possess these characteristics. Hence, alveoli are the real respiratory part of the lung in a human being. There are as many as 300 to 400 million alveoli in a lung of an adult human. The total alveolar surface through which interchange of gases takes place far exceeds the general body surface and has been estimated to be about 140 square metres. Moreover, the total capillary surface area available for gaseous exchange is about 125 square metres.