D. Bronchioles: These are branches of the smallest bronchi. The largest bronchioles differ from the smallest bronchi only by the absence of cartilage and glands in their walls. Large bronchioles are lined by typical respiratory epithelium; as they branch further, the epithelial height and complexity decrease to simple ciliated columnar or cuboidal. Each bronchiole gives rise to 5-7 terminal bronchioles.

E. Terminal Bronchioles: The smallest components of the conducting portion of the respiratory system, these are lined by ciliated cuboidal or columnar epithelium and have few or no goblet cells. The lining here also includes dome-shaped cilia-free Clara cells, whose cytoplasm contains glycogen granules, lateral and apical Golgi complexes, elongated mitochondria, and a few secretory granules. The function of these cells is unclear. Each terminal bronchiole branches to form 2 or more respira tory bronchioles.

F. Respiratory Bronchioles: These are the first part of the respiratory portion, with a cuboidal epithelial lining which resembles that of the terminal bronchioles but which is interrupted by thin-walled saccular evaginations called alveoli. The number of alveoli increases as the respiratory bronchioles proceed distally. As the alveoli increase in number, the cilia decrease until they disappear. Goblet cells are absent.

G. Alveolar Ducts: These are simply the distal extensions of the respiratory bronchioles where the alveoli are so dense that the wall consists almost entirely of these sacs, and the lining has been reduced to small knobs of smooth muscle covered by cilia-free simple cuboidal cells.

H. Atria and Alveolar Sacs: Atria are the distal terminations of alveolar ducts. The arrangement is comparable to a long hallway (alveolar duct) leading to a rounded foyer (atrium). The foyer has small doorways leading to some small rooms (alveoli), but also has 2 or more larger doorways leading into short, dead-end hallways (alveolar sacs). The short hallways are also lined by small rooms (alveoli). Put simply, the difference between atria and alveolar sacs is that the atria open into alveolar ducts, alveoli, and alveolar sacs, while the alveolar sacs open only into alveoli and atria.

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