Plate 11.228 Respiratory Bronchiole, Duct, and Alveoli

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.

Respiratory bronchiole: A branch of the terminal bronchiole. Epithelium low columnar to low cuboidal. Cilia present in the larger tubes only. Thin supporting wall of collagenous and elastic fibers and smooth muscle. Differs from the terminal bronchiole in having alveoli as outpouchings of its wall. Accompanying arterioles and venules are seen in the wall of the bronchiole. Note that large arterioles and venules are not seen at a level below the respiratory bronchiole.

Alveolar duct: Arises by branching of respiratory bronchioles; wall made up of alveolar sacs and alveoli. Lining epithelium is reduced to flattened cells with occasional cuboidal cells.

Alveolar sacs: Cluster of alveoli that open into the lumen of the alveolar duct. Individual alveoli are lined by thin squamous cells (Pneumocyte 1) and cuboidal cells that bulge into the alveolus (Pneumocyte II). The latter cell is responsible for the production of surfactant, which maintains the configuration and stability of the alveolus and plays a role in fluid transport across the alveolocapillary membrane. Site of respiratory exchange.

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