Bronchioler

Bronchioler is not a standard term in modern anatomy, but it is sometimes encountered as a reference to the bronchioles, the small airways of the lungs. Bronchioles form the distal portion of the airway tree, bridging the bronchi with the alveolar ducts and sacs where gas exchange occurs. The bronchiolar system includes terminal bronchioles, and in some regions the respiratory bronchioles, which bear alveolar outgrowths.

Anatomy and histology: Bronchioles lack cartilage and glands in their walls. They are lined by a ciliated epithelium that transitions from the thicker, mucus-producing epithelium of the larger airways to a simpler cuboidal or low-columnar type in the distal segments. Goblet cells are scarce or absent in the distal bronchioles. A muscular layer of smooth muscle encircles the bronchiolar wall, allowing autonomic regulation of airway caliber. Terminal bronchioles contain club (Clara) cells that secrete protective substances and contribute to detoxification. Respiratory bronchioles, when present, have alveoli along their walls and participate to a limited extent in gas exchange.

Function: The bronchioles primarily conduct air to the gas-exchange regions of the lung and regulate airflow resistance through smooth muscle tone. In the respiratory bronchioles, epithelial surfaces adjoin alveolar tissue, enabling minor alveolar ventilation and exchange.

Development and clinical significance: Bronchiolar development begins during fetal life and continues after birth as the lung grows. Infections and inflammation can involve bronchioles, causing bronchiolitis, especially in infants (often due to RSV). Chronic conditions such as asthma and chronic obstructive pulmonary disease can feature bronchiolar remodeling and hyperreactivity. Severe injury may lead to bronchiolitis obliterans, a scar-mediated obstruction of small airways.

Terminology: Bronchioles are the standard term for these small airways; bronchiolar describes their relating to bronchioles. The form bronchioler may appear in older or nonstandard texts.