How many alveoli are found in an adult human lung?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

An adult human lung contains approximately 480 million alveoli.1

These small, polyhedral air spaces are the primary sites of gas exchange within the respiratory system, where oxygen enters the blood and carbon dioxide is removed. While some sources may reference an estimate of 300 million, more recent anatomical data indicates the average count is approximately 480 million. These structures are densely packed at the ends of the bronchioles, forming clusters known as alveolar sacs.213