Question

respiratory system
what is the purpose of alveoli?
they tell the brain when to breathe.
they directly help the heart beat.
they allow gases to dissolve into the blood stream.
they make sound as air passes over them.

Explanation:

Alveoli are tiny air - sacs in the lungs. Their main function is gas exchange. Oxygen from the inhaled air dissolves into the bloodstream in the alveoli, and carbon dioxide from the blood is released into the alveoli to be exhaled.

Answer:

They allow gases to dissolve into the blood stream.