bronchiolo

Bronchiolo, singular for bronchiole, refers to the small airways of the lungs that branch from the larger bronchi and form part of the bronchial tree. They are part of the conductive airway system and lack cartilage and glands, distinguishing them from the larger bronchi.

Anatomy and subdivisions: Bronchioles branch from the terminal bronchioles into respiratory bronchioles, then into alveolar ducts and sacs. In humans, bronchioles are divided into terminal bronchioles, which conduct air, and respiratory bronchioles, where the pathway approaches regions capable of gas exchange. Their walls contain smooth muscle that can constrict or dilate to regulate airway resistance.

Epithelium and cells: The lining transitions from ciliated pseudostratified epithelium in larger airways to a simpler, often cuboidal epithelium in the distal bronchioles. Club cells (also called Clara cells) are common in the terminal bronchioles and secrete protective proteins; goblet cells decrease in density as the airways narrow. Cilia are retained through much of the bronchiolar region.

Function: Bronchioles serve to conduct, condition, and regulate the flow of air to the gas-exchange regions. The smooth muscle in their walls allows autonomic regulation of airway caliber, affecting airflow. Gas exchange begins in the respiratory bronchioles and continues in the adjacent alveolar structures, with defense supported by resident macrophages and other immune components.

Clinical relevance: Inflammation or obstruction of bronchioles is termed bronchiolitis, common in infants and caused by viral infections or irritants. Chronic bronchiolar changes are seen in asthma and certain forms of chronic obstructive pulmonary disease, where airway narrowing contributes to symptoms. Treatments often target smooth muscle tone and inflammatory pathways affecting these small airways.