The alveoli

There are approximately 300 million alveoli in each lung. Their total surface area is 40-80 m².The epithelial lining consists largely of type I pneumocytes. These cells have an extremely attenuated cytoplasm, and thus provide only a thin barrier to gas exchange. They are derived from type II pneumocytes. Type I cells are connected to each other by tight junctions that limit the fluid movements in and out of the alveoli. Type II pneumocytes are slightly more numerous than type I cells but cover less of the epithelial lining. They are found generally in the borders of the alveolus and contain distinctive lamellar vacuoles, which are the source of surfactant. Macrophages are also present in the alveoli and are involved in the defence mechanisms of the lung.

The pores of Kohn are holes in the alveolar wall allowing communication between alveoli of adjoining lobules.