Bronchiolen

Bronchiolen is the Dutch term for the small airways of the lungs known in English as bronchioles. They are the passages that follow the larger bronchi and precede the alveolar ducts, forming an essential part of the bronchiolar tree. The bronchiolen are commonly divided into two portions: the terminal bronchiolen, which are the last purely conducting airways, and the respiratory bronchiolen, which have airway walls bearing alveoli and thus are the beginning of the gas-exchange region.

Histology and structure vary along the length of the bronchiolen. Cartilage and submucosal glands are absent, and the epithelium changes from ciliated pseudostratified columnar in larger airways to a simple cuboidal type in terminal bronchiolen. Goblet cells are largely absent in this region. Club cells (Clara cells) line many terminal bronchiolen and contribute to detoxification and maintenance of the airway lining. The walls contain smooth muscle, allowing regulation of airway caliber. In respiratory bronchiolen, alveolar outgrowths occur along the walls, marking the transition toward true gas exchange.

Functionally, the bronchiolen conduct and condition inspired air, support mucociliary clearance, and regulate airflow via smooth muscle tone. Gas exchange begins in the respiratory bronchiolen and continues through the subsequent alveolar ducts and sacs.

Clinical relevance includes bronchiolitis, an inflammation of the bronchiolen often seen in viral infections in infants, and bronchiolitis obliterans, a fibrotic, irreversible small-airway obstruction. Small-airway disease is also a feature of various chronic lung conditions, such as asthma and chronic obstructive pulmonary disease.