What are the tiny air sacs in the lungs called?

The tiny air sacs in the lungs are called alveoli. Each alveolus is a microscopic, cup-shaped structure where oxygen enters the blood and carbon dioxide leaves it to be exhaled.

Alveoli sit at the ends of branching airways inside clusters called alveolar sacs. Their extremely thin walls are surrounded by dense networks of pulmonary capillaries. This arrangement creates a short blood-air barrier, allowing gases to diffuse efficiently between inhaled air and circulating blood.

A typical pair of adult human lungs contains roughly 480 million alveoli, with a combined gas-exchange surface area of about 70–80 square metres. Elastic fibres help the sacs expand during inhalation and recoil during exhalation. Type II alveolar cells produce pulmonary surfactant, a substance that reduces surface tension and helps prevent the sacs from collapsing.

Alveoli are not the same as bronchi or bronchioles: those tubes carry air, while alveoli perform the actual exchange of gases. Conditions such as pneumonia, emphysema, pulmonary edema, and some forms of respiratory distress can impair this delicate exchange surface.