Answer:
The alveoli are sacks coated on their inner wall by white and sticky liquid, they can be more than a millimeter in diameter and surface active agent. In them there is the exchange of gases between O2 and CO2.
They are evaginations of the epithelium of the air ducts with a single opening for the gases to exit and enter, controlled by the action of a smooth muscle sphincter. Its walls, called alveolar septa, provide a large increase in the exchange surface.
