The trachea divides into two bronchi (Fig. 4), one for each lung. The bronchi branch off into smaller bronchioles.
Bronchioles have a diameter of less than 1 millimetre. In bronchioles, there are no cartilaginous rings, but cartilage discs make them more rigid. The bronchiolar epithelium has no cilia and does not produce mucus. There is an increased amount of smooth muscle around the bronchioles (Fig. 5). When they contract, these muscle cells narrow the diameter of the bronchioles, so that the volume of air entering and leaving the alveoli is regulated (this is covered later).
Bronchioles branch off to the terminal bronchioles, which have a diameter of less than 0.5 mm. The terminal bronchioles lead to the respiratory bronchioles. The respiratory bronchioles terminate in a cluster of bubble-like structures calledalveoli or alveolar sacs (Fig. 5).