An alveolus is a tiny air sac formed at the tip of the lungs' smallest airways, the bronchioles. The primary function of the alveoli is the exchange of carbon dioxide for oxygen. Tissues within the alveoli also carry out secondary functions, such as the production of hormones, enzymes, and pulmonary surfactant. Secondarily, the alveolus is the site where inhaled substances, such as pathogens, drugs, or other chemicals, are usually processed.
The pulmonary alveolus is a two-tiered structure. A network of elastic fibers and capillaries, resembling a rounded basket, forms the exterior structure of each alveolus. These capillaries branch from both the pulmonary artery and the pulmonary vein. Oxygen-depleted blood is carried by the pulmonary artery from the heart to the lungs, while the pulmonary vein carries oxygen-rich blood away from the lungs and back to the heart. Known as the capillary bed, the blood vessels surrounding the alveoli are not necessary only for gas exchange, but also to provide elastic support to the delicate inner lining of the alveoli.
The alveolar lining, or epithelium, is a thin tissue that forms the interior of the alveoli. This tissue is coated with a moist film of pulmonary surfactant, a liquid substance that is produced within the alveoli. Pulmonary surfactant aids in diffusion and prevents alveolar collapse during exhalation. Alveolar epithelium is made of two distinct types of cells, which, together with the pulmonary surfactant, form the respiratory surface of the lungs.
Flat type I cells cover most of the surface area of the epithelium in a tile-like pattern. Adjacent cells are joined by tight junctions, creating a membrane that will only allow the very smallest molecules to pass between them. These cells facilitate the diffusion of respiratory gases to and from the capillary bed by allowing oxygen and carbon dioxide molecules, dissolved in surfactant, to pass through the intracellular spaces.
Type II alveolar cells are more numerous than the type I cells, yet occupy a smaller area of the alveolar lining. Cuboidal in shape, these cells perform a number of important tasks within the alveolus. Chief among these tasks is producing the pulmonary surfactant that is necessary for gas exchange. Additionally, type II cells synthesize a number of substances important to healthy lung function. If needed, type II cells also have the ability to change form to replace damaged type I cells.
Also inhabiting the epithelium are large, round cells known as macrophages. These cells roam the alveolar space freely, ingesting fine particulate matter such as dust, tar, and pathogens. Once they are full, macrophages retire to the connective tissue outside the alveoli. In smokers or other individuals exposed to a high level of pollutants, accumulated macrophages appear as a heavy, black residue surrounding the lung tissue.
Each pulmonary alveolus measures, on average, ten thousandths of an inch (about 250 micron), or just over twice the thickness of a human hair. At birth, an average human has approximately 200,000,000 alveoli. By adulthood, this number will typically have doubled. Despite their tiny size, this translates to a respiratory surface area of over 1,500 square feet (143 square meters.)