Question

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

Explanation:

Alveoli are tiny air - sacs in the lungs. Their main function is gas exchange, allowing oxygen to dissolve into the bloodstream and carbon dioxide to be released from the bloodstream.

Answer:

They allow gases to dissolve into the blood stream.