Clara cells are most concentrated in the smaller bronchioles, and they appear to produce a surfactant-like compound. A prominent layer of circularly arranged smooth muscle is characteristic of all bronchioles. The last purely conducting portion of the respiratory pathway. Circularly arranged smooth muscle is present outside the epithelium. These have alveoli in their walls functioning in gaseous exchange between air and blood.
Smooth muscle acts as sphincters at alveolar openings.
They are simply composed of rolls of lung alveoli. The interalveolar walls contain a profusion of capillary of the continuous type, as well as an abundance of elastic and reticular fibers. These are the blind ends of the alveolar ducts. Like the latter, they are composed of alveoli. This reveals relatively little about the lining cells of the alveoli.