‘Final branches of the respiratory tract and act as the primary gas exchange units of the lung.

Oxygen diffuses through the alveolar epithelium, the interstitial space and the capillary endothelium to reach the blood.

CO2 follow the reverse course as does oxygen.

There are 2 types of epithelial cells: type I pneumocyte cells are flattened, platelike cells with long extensions that thinly spread out over the alveolar walls and comprise about 95% of the surface, type II pneumocyte cells are rounded and more compacted cells that are responsible for producing surfactant.

Type II cells are the main cells involved in repair of the alveolar epithelium when type I cells are destroyed.

Alveolar macrophages are loosely attached to the epithelial cells or are free in the alveolar space and belong to the mononuclear macrophage system.

The walls are not solid but perforated by pores of Kohn, that allow the passage of bacteria and exudative secretions between alveoli.

Adjacent to the alveoli cell membrane is the surfactant layer.

There are about 300 million alveoli in each lung.

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