Alveolus

An alveolus (plural alveoli) is a microscopic, cup-shaped air sac in the lung. The airways branch again and again into ever smaller bronchioles, which end in alveolar ducts and grape-like clusters of alveoli called alveolar sacs. Each alveolus is only about 0.2 millimeters across, but the lungs contain well over a hundred million of them, and each is wrapped in a dense net of capillaries.

The alveolar wall is mostly made of type I alveolar cells, flat cells so thin that gases cross them easily. Together with the capillary wall they form the respiratory membrane, less than a micrometer thick. Oxygen diffuses from the air into the blood and carbon dioxide diffuses the other way, each moving down its own concentration gradient. Scattered type II alveolar cells secrete surfactant, a fluid that lowers surface tension so the alveoli do not collapse when we breathe out. Alveolar macrophages patrol the surface, removing dust and microbes, and small pores between neighboring alveoli help even out air pressure.

Because gas exchange depends on this huge, thin surface, damage to the alveoli is at the heart of many lung diseases. In pneumonia they fill with fluid, and in emphysema their walls break down, shrinking the area available for gas exchange.