part of part of Homo sapiens respiratory tube lung pulmonary alveolar duct http://upload.wikimedia.org/wikipedia/commons/8/8f/Gray975.png ductus alveolaris UBERON:0002173 respiratory alveolar duct MA:0000417 The respiratory conducting tubes distal to the respiratory bronchiole that lead to the alveolar sacs and the alveoli. NCIT:C32056 UMLS:C0391892 EMAPA:32694 Wikipedia:Alveolar_duct FMA:7341 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 alveolar duct alveolar system