(i) Lungs provide very large surface area for the exchange of gases. Total alveolar surface area is nearly 100 m2 whereas total surface area of skin is around 1.6 m2 only.
(ii) Alveoli are lined by thin highly permeable membranous wall. These are surrounded by many blood capillaries.
(iii) Endothelium of blood capillaries and membranous walls of alveoli are highly permeable to respiratory gases.
