Alveoli are the site of gaseous exchange. The number of alveoli in both the lungs is about 75 crores which have a surface area of About 100 square meters more than the area of a tennis court. The diffusion membrane is made of 3 layers,

● The thin simple squamous epithelium of alveoli

● The endothelium of pulmonary capillaries

● And the basement membrane in between them.

Combines thickness is less than 1 mm, this much thickness is essential for the easy exchange of gases.