Alveoli

The alveoli are microscopic, balloon-like air sacs that sit at the very ends of the airways in the lungs. They are the functional units of the respiratory system, the actual site where oxygen enters the blood and carbon dioxide leaves it. Each lung contains hundreds of millions of alveoli, giving the lungs an enormous internal surface area for gas exchange.

Gross Structure and Location

Alveoli cluster in grape-like bunches called alveolar sacs at the ends of the smallest airways (alveolar ducts and respiratory bronchioles). Each alveolus is roughly 200 micrometers across and has a very thin, elastic wall. That wall is lined mainly by two cell types: flat type I pneumocytes, across which gases diffuse, and rounded type II pneumocytes, which produce surfactant, a soapy substance that lowers surface tension and keeps the sacs from collapsing. A dense mesh of pulmonary capillaries wraps around each alveolus. The alveolar wall and the capillary wall together form the extremely thin respiratory membrane through which gases pass.

Key Functions

The alveoli exist to maximize gas exchange. Oxygen in inhaled air diffuses across the thin respiratory membrane into the blood in the surrounding capillaries, where it binds to hemoglobin in red blood cells. At the same time, carbon dioxide diffuses out of the blood into the alveoli to be exhaled. The combined surface area of all the alveoli is very large, which allows efficient exchange with each breath. Surfactant and the alveoli's elastic recoil are essential for keeping the sacs open and for normal breathing mechanics. Resident immune cells called alveolar macrophages patrol the sacs and engulf inhaled particles and microbes.

Clinical Relevance

Alveolar damage underlies several serious conditions. In emphysema (a form of COPD), alveolar walls are destroyed, reducing surface area and trapping air. In pneumonia, alveoli fill with fluid and inflammatory cells, impairing oxygen uptake. Acute respiratory distress syndrome (ARDS) causes widespread alveolar injury and flooding. In premature newborns, too little surfactant leads to respiratory distress syndrome, which is why surfactant replacement therapy can be lifesaving.

This page is educational and not a substitute for professional medical advice.