Bronchioles are small airways that branch multiple times from the segmental bronchi all the way to the alveoli, or air sacs, of the lungs. They are small, hollow, muscular tubes that lack hyaline cartilage. The smallest bronchioles are subcategorized as terminal bronchioles, followed by the smallest respiratory bronchioles.
Terminal bronchioles are the final portion of the conducting zone and lead directly into the respiratory zone where gas exchange occurs.
Respiratory bronchioles turn into alveolar ducts culminating in alveolar sacs, which contain clusters of interconnected alveoli.
Figure 1: The bronchioles of the bronchial tree, and a cross-section of a bronchiole.