The terminal bronchiole is the connecting link between the conducting portion and the respiratory portion. In many mammals there is a short portion called the respiratory bronchiole whose walls consist partly of alveoli.

HA17 is a low magnification image of a section of mouse lung. T labels the terminal bronchiole cut in longitudinal (middle) and cross-section (right). Where the terminal bronchiole ends, the epithelium changes its appearance, and an alveolar duct begins. The airway still has the form of a tube, but the wall is predominantly occupied by alveoli or alveolar sacs. Pulmonary vessels extend out to the ends of the terminal bronchioles.