The trachea divides into two tubes called bronchi, which enter each lung respectively. The bronchi divides further into secondary bronchi, tertiary bronchi, and then bronchioles. The bronchioles further divide into small air sacs called alveoli.
The alveoli are small sacs of air so thin walled that their walls are just one cell thick. These provide an interface for the diffusion of respiratory gases in and out of the blood stream, because the air sacs containing respiratory gases are in contact with the blood stream of blood vessels.

Structures involved

The Bronchi-