You are looking at a structure in the respiratory tract that is filled with small sacs or air pockets. What structure are you looking at?

Alveoli.

Explanation:

Alveoli are the hollow cavity present in the lung parenchyma. Alveoli are the main site of exchange of gases in human beings.

The alveoli has structure like a sac or air pockets that filled with the gas. The basic unit of the ventilation is alveoli. The epithelial layer covers the surface of alveoli.

Thus, the correct answer is alveoli.