Alveoli

About the Alveoli

Alveoli are tiny, thin walled air sacs clustered at the ends of the respiratory bronchioles, and the lungs contain hundreds of millions of them, together providing a very large surface area for gas exchange. Each alveolus is surrounded by a dense network of capillaries, and the barrier between air and blood is extremely thin, allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out. The alveolar wall contains two main cell types: thin type I cells that form most of the surface for gas exchange, and type II cells that produce surfactant, a mixture of lipids and proteins that reduces surface tension and prevents the alveoli from collapsing. Premature babies born before their lungs make enough surfactant can develop respiratory distress syndrome. Alveolar macrophages remove inhaled particles and microbes. Diseases that damage the alveoli include emphysema, in which alveolar walls are destroyed, pneumonia, in which they fill with fluid and inflammatory cells, and pulmonary fibrosis.