Respiratory bronchioles

Respiratory bronchioles are the final division of the bronchioles within the lung.  They are a continuation of the terminal bronchioles and are approximately 0.5mm in size . They are comprised of simple cuboidal epithelium and contain a thin layer of smooth muscle and elastic fibers . Importantly, alveoli start to open off them and thus are the first bronchioles which allow gas exchange to occur. The respiratory bronchioles then split into alveolar ducts which in turn terminate into alveolar sacs and individual alveoli.