Alveolar ducts are tiny ducts that connect the respiratory bronchioles to alveolar sacs, each of which contain a bunch of alveoli (the balls). They are tiny ducts of the branching airways that fill the lungs. Each lung holds approximately 1.5 to 2 million of them. These are also often known as the final branches of the human bronchial tree. The tubules divide into two or three alveolar sacs at the distal end. They are formed from the confluence openings of several alveoli. Distal terminations of alveolar ducts are atria which then end in alveolar sacs. In human anatomy, respiratory bronchioles exists proximal to the alveolar ducts. 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.