Alveolar
| Pulmonary alveolus | |
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The alveoli
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| Details | |
| System | Respiratory system |
| Identifiers | |
| Latin | alveolus pulmonis |
| Code | TH H3.05.02.0.00026 |
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Anatomical terminology
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An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is a hollow cavity found in the lung parenchyma, and is the basic unit of respiration. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli 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. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen.
The alveoli are located in the respiratory zone of the lungs, at the ends of the alveolar ducts and alveolar sac, representing the smallest units in the respiratory tract. They provide total surface area of about 75 m2.
A typical pair of human lungs contain about 700 million alveoli, producing 70m2 of surface area. 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 micrometres, with an increase in diameter during inhalation.
The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn. The alveoli contain some collagen and elastic fibres. The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air.
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