How Many Alveoli Are There in
the Human Lung?

Answer:

In both lungs of an adult there are about 300 million alveoli.

If they were all flattened out their surface area would be about 50 sq metres when the lungs have breathed out, and up to 150 square metres when they breathe in.

The total area of the capillary blood vessels around all the alveoli is about 100 square metres.