The essential features of the alveolar epithelium as a surface over which gas exchange takes place.

Epithelila cells make up the walls of alveoli.

The alveoli are surrounded by capillaries, this means there is a constant flow of blood that takes oxygen away from the area of diffusion, and the lungs bring a constant supply of oxygen to be diffused on the other side; the opposite is true for CO2- this keeps a big difference in concentration gradient to speed up diffusion.

Both alveoli and capillaries have very thin walls making the diffusion distance short (so diffusion happens more quickly.)

Because there are so many alveoli there is a very large total surface area fro gasses to diffuse through (quicker diffusion.)

There is mucus lining the alveoli which helps gasses to diffuse across.