Slide C
Respiratory bronchioles are lined partly by bronchiolar epithelium and partly by alveoli. These structures open to alveolar ducts, which terminate in alveolar sacs and alveoli.

Slide D
The alveolar walls are normally thin, and only a few nuclei are evident. These are the nuclei of capillary endothelial cells, interstitial cells, and perhaps a rare mononuclear cell. The alveolar epithelial calls are normal inconspicuous.