bronchioli

Bronchioles, also called bronchioli in some texts, are the small airways of the lungs that conduct air from the segmental bronchi toward the regions where gas exchange occurs. They lack cartilage and glands in their walls, and their diameter is modulated by smooth muscle, allowing regulation of airway resistance. Autonomic nerves and local mediators influence constriction and dilation.

Anatomy and histology

Bronchioles branch from larger bronchi and progress from terminal bronchioles to respiratory bronchioles. Terminal bronchioles are lined predominantly by simple cuboidal epithelium with club (Clara) cells and ciliated cells; goblet cells are sparse or absent, and goblet-cell–derived mucus production diminishes in the smaller airways. The walls contain relatively thick smooth muscle, contributing to airway tone. Respiratory bronchioles are transitional passages that bear occasional alveoli along their walls, marking the beginning of gas exchange. In the transition from terminal to respiratory bronchioles the epithelium becomes increasingly simple and the density of mucus-producing cells decreases.

Function

Bronchioles primarily conduct air to the gas-exchange regions while enabling control of airway resistance through smooth muscle. Mucociliary clearance is most effective in larger bronchioles; in the deepest portions of the bronchiolar tree, gas exchange begins in the respiratory bronchioles and associated alveolar structures.

Clinical relevance

Inflammation of the bronchioles is known as bronchiolitis, commonly caused by viral infections in children and by other agents in adults. Chronic diseases can involve small airways, including bronchiolitis obliterans, asthma-related bronchoconstriction, and certain forms of chronic obstructive pulmonary disease, where small-airway obstruction contributes to symptoms and airflow limitation.