The alveoli are small sacs at the end of the bronchioles in the lungs that exchange carbon dioxide for oxygen. They also produce hormones, enzymes, and surfactant, and process inhaled substances. The alveoli consist of a network of capillaries and elastic fibers, with a thin epithelium lining coated in surfactant. Type I and II cells and macrophages make up the epithelium, with type II cells producing surfactant and other substances. The average person has 400 million alveoli, providing a breathing surface area of over 1,500 square feet.
An alveolus is a small sac of air 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. The tissues within the alveoli also perform secondary functions, such as the production of hormones, enzymes, and pulmonary surfactant. Second, the alveolus is the site where inhaled substances, such as pathogens, drugs, or other chemicals, are usually processed.
The pulmonary alveolus is a two-level structure. A network of elastic and capillary fibers, similar to a rounded basket, forms the external 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 only needed for gas exchange but also to provide springy support to the delicate inner lining of the alveoli.
The alveolar lining, or epithelium, is a thin tissue that forms the inside of the alveoli. This tissue is coated with a moist film of pulmonary surfactant, a liquid substance produced inside the alveoli. Pulmonary surfactant promotes diffusion and prevents alveolar collapse during expiration. The alveolar epithelium consists of two distinct cell types which, together with the pulmonary surfactant, form the respiratory surface of the lungs.
Type I flat cells cover most of the surface of the epithelium in a tile-like pattern. Adjacent cells are joined by tight junctions, creating a membrane that will allow only the 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 the surfactant, to pass through the intracellular spaces.
Type II alveolar cells outnumber type I cells, but occupy a smaller area of the alveolar lining. Cube-shaped, these cells perform a number of important tasks within the alveolus. Chief among these tasks is to produce the pulmonary surfactant needed for gas exchange. In addition, type II cells synthesize a number of substances that are important for healthy lung function. If necessary, type II cells also have the ability to change shape to replace damaged type I cells.
Large, round cells known as macrophages also inhabit the epithelium. These cells roam freely in the alveolar space, ingesting fine particles such as dust, tar, and pathogens. Once full, the macrophages retreat into the connective tissue outside the alveoli. In smokers or other individuals exposed to a high level of pollutants, the 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 microns), or slightly more than twice the thickness of a human hair. At birth, an average human being has approximately 200,000,000 alveoli. By adulthood, this number will typically have doubled. Despite their small size, this translates into a breathing surface area of over 1,500 square feet (143 square meters).
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