alveoli

Alveoli are microscopic air sacs at the ends of the respiratory tree within the lungs. They are the primary site of gas exchange, allowing oxygen from inspired air to diffuse into the blood and carbon dioxide to diffuse from the blood into the air to be exhaled. In adults, the lungs contain roughly 480 million alveoli, creating a total surface area of about 70 to 100 square meters, which supports rapid gas transfer.

Each alveolus is a small sac lined by a very thin epithelium and surrounded by a dense capillary network. The alveolar wall is composed mainly of type I pneumocytes, which cover most of the surface area and form the thin barrier for diffusion, and type II pneumocytes, which secrete surfactant to reduce surface tension and can proliferate to repair the epithelium. Alveolar macrophages reside in the airspaces to clear inhaled particles and pathogens. The alveoli are connected to relatively small airways by alveolar ducts, and pores of Kohn allow collateral ventilation between neighboring sacs.

Gas exchange occurs across the alveolar-capillary membrane, a structure that includes the alveolar epithelium, the fused basement membranes, and the capillary endothelium. The membrane is extremely thin, facilitating diffusion; oxygen diffuses from the air into the blood, while carbon dioxide diffuses out of the blood into the alveolar air. Surfactant produced by type II cells lowers surface tension, preventing alveolar collapse during exhalation.

Clinical relevance: alveolar dysfunction or destruction, as seen in emphysema or acute respiratory distress syndrome, impairs gas exchange and oxygenation.