HA19 illustrates an alveolar duct that begins at the end of a terminal bronchiole (T) at the upper left. The arrowheads indicate small clusters of simple cuboidal epithelium with cilia. The wall of the duct consists mainly of openings into alveoli. Atria (a) communicate with several alveolar sacs. Each alveolar sac has a cluster of alveoli that share a common chamber.