This image shows a section of inflated lung containing a respiratory bronchiole.   Although the walls of respiratory bronchioles contain respiratory epithelium (which prevents gas exchange), note that there are also sections that contain thin-walled alveoli (air sacs) through which gas exchange can take place.   The distal ends of respiratory bronchioles open into alveolar ducts whose walls are formed by a series of alveoli lying adjacent to one another.  Coming off the alveolar ducts are groups or clusters of alveoli that form alveolar sacs.