The bronchioles (/ˈbrɑːŋkioʊls/ BRONG-kee-ohls) are the smaller branches of the bronchial airways in the lower respiratory tract.

Bronchioles are approximately 1 mm or less in diameter and their walls consist of ciliated cuboidal epithelium and a layer of smooth muscle.

A decrease in diameter is called bronchoconstriction, which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated by histamine, parasympathetic nerves, cold air, chemical irritants, excess mucus production, viral infections, and other factors to decrease air flow.

The bronchioles are histologically distinct from the bronchi in that their walls do not have hyaline cartilage and they have club cells in their epithelial lining.

Instead, they rely on elastic fibers attached to the surrounding lung tissue for support.

The inner lining (lamina propria) of these bronchioles is thin with no glands present, and is surrounded by a layer of smooth muscle.

Terminal bronchioles are lined with simple ciliated cuboidal epithelium containing club cells.

Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles.

[6] Bronchospasm, a potentially life-threatening situation, occurs when the smooth muscular tissue of the bronchioles constricts, severely narrowing their diameter.