The respiratory bronchioles have single alveoli off their walls. The epithelium is ciliated cuboidal epithelium and contains some secretory cells called clara cells. The respiratory bronchioles lead into alveolar ducts, (which are surrounded by smooth muscle, elastin and collagen), which lead into the alveolar sacs.

Do all bronchioles have alveoli?

Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with simple cuboidal epithelium containing club cells.

What do bronchioles contain?

The bronchioles contain a continuous circular layer of smooth muscle that is believed to be regulated by the parasympathetic nervous system. The mucosa is continuous with that of the bronchi.