Diffusion of gases occur in the alveolar region only and not in the other part of respiratory system. Why?
For efficient exchange of gases, respiratory surface must have certain characteristics such as
(i) it must be thin, moist and permeable to respiratory gases.
(ii) it must be large.
(iii) it must be highly vascular.
Only alveolar region has these characteristics. Thus, diffusion of gases occur in this region only.