Alveoli are the tiny air sacs in the lungs where oxygen enters the blood. Each alveolus is a thin-walled, cup-shaped cavity surrounded by a dense network of pulmonary capillaries. Together, the enormous number of alveoli creates a broad surface for rapid gas exchange between inhaled air and circulating blood.
Oxygen moves by diffusion across the extremely thin blood–air barrier into capillaries, where it binds mainly to hemoglobin in red blood cells. At the same time, carbon dioxide travels in the opposite direction—from the blood into the alveolar air—and is exhaled. Surfactant, produced by specialized type II cells, helps prevent the alveoli from collapsing by reducing surface tension.
Bronchioles deliver air toward the respiratory zone but are not the main exchange sacs. Villi are projections in the small intestine that absorb nutrients, and cilia are microscopic hair-like structures that move mucus in many airways. Alveoli are characteristic of mammalian lungs; other vertebrates use different structures for gas exchange.