From the tertiary bronchi, air is conducted to the alveoli (or air sacs) in the lung lobules by a series of small, branching tubules called bronchioles.
The branching produces progressively smaller diameter tubules, each measuring less than 3 millimeter.
A single lobule bronchiole (or preterminal bronchioles), conducts air in and out of a lung lobule. After it enters a lobule, a lobule bronchiole divides into approximately six terminal bronchioles, and each terminal bronchiole gives rise to one to three respiratory bronchioles. From here, air finally travels in and out of the alveoli via alveolar ducts.
