An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange surface. The blood brings carbon dioxide from the rest of the body for release into the alveoli, and the oxygen in the alveoli is taken up by the blood in the alveolar blood vessels, to be transported to all the cells in the body.
The alveoli are located in the respiratory zone of the lungs, at the distal termination of the alveolar ducts and atria, forming the termination point of the respiratory tract. They provide total surface area of about 75 m2.
The alveoli contain some collagen and elastic fibres. The elastic fibers allow the alveoli to stretch as they fill with air when breathing in. They then spring back during breathing out in order to expel the carbon dioxide-rich air.
Each human lung contains about 300 million alveoli. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 to 300 micrometres, with an increase in diameter during inhalation.
The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn.
There are three major alveolar cell types in the alveolar wall (pneumocytes):
- Type I (Squamous Alveolar) cells that form the structure of an alveolar wall
- Type II (Great Alveolar) cells that secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate, thereby increasing the capability to exchange gases. Surfactant is continuously released by exocytosis. It forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed primarily of dipalmitoyl phosphatidylcholine.
- Macrophages that destroy foreign material, such as bacteria.
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