<unk> s are the tiny end<unk> s of the branching airways that fill the lungs. Each lung holds approximately 1.5 to 2 million of them. The tubules divide into two or three<unk>  sacs at the distal end. They are formed from the confluence openings of several alveoli. Distal terminations of<unk> s are atria which then end in<unk>  sacs.
In human anatomy, respiratory bronchioles exists proximal to the<unk> s. The epithelial lining consists of smooth muscle knobs covered by nonciliated, simple cuboidal cells. The smooth muscle constricts under parasympathetic innervation and relax under sympathetic innervation.
Alveolar duct
Bronchiole
Bronchial veins
Tertiary bronchus
Secondary bronchus
Bronchus
Aortopulmonary Window
Tracheal bronchus