Plate 11.230 Alveolus

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Refer to Figure 11A which can be used in conjunction with this plate in order to follow the structural changes that occur from the respiratory bronchiole to alveolar ducts to alveolar sacs where gaseous exchange takes place.

The partition between adjacent alveoli is shown in this plate. Note that capillaries form a major part of the partition, the very thin alveolar wall and the alveolar cells. The alveolar wall has been shown by electron microscopy to be composed of an epithelial cell (Type I or Type II), basement membrane (basal lamina), and an endothelial cell. The Type II alveolar cells (also known as great alveolar cells or septal cells) are cuboidal in shape and appear to have empty vacuoles in their cytoplasm. The vacuoles, as shown by electron microscopy, frequently contain osmiophilic bodies with concentric lamellae (cytosomes). Cytosomes are believed to contribute to the pulmonary surfactant that coats the alveolar epithelium in order to reduce the surface tension and keep the alveoli from collapsing. Type I, squamous alveolar cells constitute 97 per cent of the respiratory epithelium at the alveolar level. The remaining 3 per cent are Type II, great alveolar cells, which produce the surfactant. Through the alveolar wall, gaseous exchange takes place between blood and air.

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