Alveoli

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, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.

Location

The alveoli are found in the respiratory zone of the lungs, providing a total surface area of about 75 m². 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.

Anatomy

Each human lung contains about 150 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 0.2-0.3 mm, 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.

Reinflation of the alveoli following exhalation is made easier by pulmonary surfactant, which is a phospholipid and protein mixture that reduces surface tension in the thin fluid coating within all alveoli. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air. The surfactant is produced by great alveolar cells (granular pneumonocytes, a cuboidal epithelia), which are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous alveolar cells (a squamous epithelium).

Great alveolar cells also repair the endotheilium of the alveolus when it becomes damaged. Insufficient pulmonary surfactant in the alveoli can contribute to atelectasis (collapse of part or all of the lung). Without pulmonary surfactant, atelectasis is a certainty; however, there are other causes of lung collapse such as trauma (pneumothorax), COPD, and pleuritis.

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