Bronchioles are defined as conducting airways less than 1 mm in diameter that lack cartilage in their walls. Bronchioles are divided into two groups. The larger (average diameter 0.5 to 1 mm) terminal (membranous) bronchioles branch from the smallest bronchi and give rise to the smaller (average diameter 0.15 to 0.2 mm) respiratory bronchioles. The terminal (membranous) bronchioles only conduct air, similar to bronchi; whereas the respiratory bronchioles both conduct air and participate in gas exchange via the alveoli in their walls. The respiratory bronchioles branch into about two more generations of respiratory bronchioles with increasing numbers of alveoli in their walls and give rise to the alveolar ducts.

Terminal bronchioles (membranous bronchioles) are the most distal generation of bronchioles that do not contain alveoli. Terminal bronchioles have a simple columnar epithelium (bronchiolar mucosa) composed of ciliated columnar cells and nonciliated Clara cells, a layer of smooth muscle, and a connective tissue adventitia. Terminal (membranous) bronchioles lack cartilage, and seromucinous glands and goblet cells are generally not observed or are minimal in the normal bronchiolar mucosa. The terminal bronchiole leads into the acinus (a functional unit composed of the structures distal to a single terminal bronchiole—its respiratory bronchioles, alveolar ducts, and alveoli). A lobule is an anatomic unit consisting of the acini of 3 to 10 terminal (membranous) bronchioles that are bounded together by the interlobular septum. As with the bronchi, the bronchioles are accompanied by branches of the pulmonary artery of approximately the same diameter.

Respiratory bronchioles have a bronchiolar wall with simple columnar to cuboidal bronchiolar epithelium and alveoli budding from their walls. The alveoli budding from the bronchiolar walls increase in numbers the higher the generation of the respiratory bronchiole. In two-dimensional longitudinal sections of glass slides, respiratory bronchioles often appear to have a bronchiolar mucosa and wall on one side of their lumen and alveolar spaces on the opposite side of their lumen. Respiratory bronchioles represent the first generation of airways in which exchange of gases occurs.