Question

why are lungs divided into very small sac-like structure called alveoli ?​

Answer 1

Lungs have alveoli because,

The alveoli are adapted to make gas exchange in lungs happen easily and efficiently. ... they give the lungs a really big surface area. they have moist, thin walls (just one cell thick) they have a lot of tiny blood vessels called capillaries.

Answer 2

In humans lungs, balloon like structures called ALVEOLI are present that increases the surface for the exchange of gases to about 100 m sq. The alveoli have single layers of cells ans are very thin to facilitate the exchange of gases.