Alveoli are the tiny air sacs where gas exchange occurs in the human lungs.
These microscopic sacs lie at the ends of the smallest airways, called respiratory bronchioles and alveolar ducts. Their thin walls sit next to a dense network of capillaries. Oxygen diffuses from inhaled air across the alveolar–capillary barrier into blood, while carbon dioxide moves from blood into the alveolar air to be exhaled.
The lungs contain hundreds of millions of alveoli, creating a very large surface area for diffusion. Their walls are mainly formed by extremely thin type I alveolar cells. Type II alveolar cells produce pulmonary surfactant, which reduces surface tension and helps prevent alveoli from collapsing.
Bronchi and bronchioles conduct air but are not the principal exchange surfaces. The alveoli must remain moist for gases to dissolve and cross their membranes. Conditions such as emphysema damage or destroy alveolar walls, reducing the available surface area and making oxygen and carbon-dioxide exchange less efficient.