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Respiratory bronchioles are the final divisions of the bronchioles within the respiratory tree and the first to permit gaseous exchange with the blood. The respiratory bronchioles are continuous with the terminal bronchioles. In the adult, approximately 150,000 main respiratory bronchioles exist in each lung; these undergo a further three divisions to produce about 500,000 respiratory bronchiole branches in each lung.
Histologically, the respiratory bronchioles are similar to the terminal bronchioles with slight changes in the epithelial layer of the wall:
Between cells there are increasingly frequent alveoli to permit gaseous exchange. Each respiratory bronchioles terminates by branching into multiple alveolar ducts.