Respiratory Membrane and Gas Exchange
Bronchioles of the lung: anatomy and function
Last update:From the tertiary bronchi, air is conducted to and from the alveoli (or air sacs) by a series of small, branching tubules called bronchioles
- The bronchioles branch many times on their way to the alveoli, and each division produces tubules that are progressively smaller in diameter (magnified here for display).
- A lobular bronchiole (or preterminal bronchiole), conducts air in and out of a pulmonary lobule (or secondary pulmonary lobule).
- After entering a pulmonary lobule, a lobular bronchiole divides into three or more terminal bronchioles.
- Terminal bronchioles measure 0.5 – 1 mm (or less) in diameter and have walls made of simple ciliated cuboidal cells, a few smooth muscle cells, and connective tissue. They are too thick for air exchange, so these tubes are considered to be the last of the conducting zone structures.
- Two or three respiratory bronchioles typically branch from each terminal bronchiole. These thin-walled tubules are the first respiratory zone structures, and they, in turn, give rise to alveolar ducts, alveoli, and alveolar sacs.
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An enlarged cross-sectional view of a bronchiole reveals the tissue layers that make up the wall.
- Ciliated simple columnar cells form the epithelial lining in the large bronchioles. In the small bronchioles, the epithelium changes to simple cuboid cells. Goblet cells and seromucous glands become less numerous with each bronchiole division.
- A ring of smooth muscle fibers surrounds the epithelium. During exhalation, these muscle fibers contract to help force air out of the bronchioles. The resulting compression causes the epithelium to fold.
- The thin-walled brochioles are attached to the surrounding elastic alveoli. This connection keeps the bronchioles from collapsing during breathing move-ments. Because they are not needed for support, cartilage plates are characteristically absent.
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- The following photomicrograph shows a more realistic depiction of the layers and structures that make up the tracheal wall. [ Epithelial folds/ smooth muscle