Question

why are lungs divided into very small sac like structures called alveoli

Answer 1

they are divided into very small sacred like structure as to increase the surface area for easily exchange of gases


hope this answer helps

Answer 2

Lungs are divided into alveoli to support easier transport of oxygen and carbon dioxide between the blood vessels and the alveoli.  

These small structures also fill the entire lung causing an organized way of oxygen transport system that happens in the body.  

They are lined by a surfactant that causes the shape of the alveolar sac to be maintained, this surfactant also helps in maintaining the surface tension and partial pressure of the gasses.