The terminal bronchioles are the branch that mark the end of the conducting division of the airflow in the respiratory system. They are about 0.5 mm or less in diameter. Terminal bronchioles further divide into alveolar ducts.

The diameter of the bronchiole plays an important part in the degree of airflow passing through them. The bronchioles change diameter to either increase or decrease airflow. An increase in diameter is called bronchodilation and is stimulated either by epinephrine or sympathetic nerves to increase airflow. A decrease in diameter is called bronchoconstriction and is stimulated by Histamine, parasympathetic nerves, cold air, chemical irritants, and other factors to decrease airflow.

Respiratory bronchioles are the beginning of the respiratory division where actual gas exchange takes place. The bronchioles or bronchioli are the passageways into the lungs by which air from the nose or mouth passes to the air sacs of the lungs. The bronchioles rely on elastic fibers attached to surrounding lung tissue for support rather than the more structured bronchi they arise from which has hyaline cartilage lining them. This hyaline cartilage is absent from the lining of the bronchioles.

In the bronchioles there are two different types of  epithelial cells lining called a simple ciliated columnar epithelium and a simple ciliated cuboidal epithelium.

Clara cells are domed shaped cells with short microvilli found in the bronchioles of the lungs. Clara cells are also known as “club cells” and “ bronchiolar exocrine cells” as of January 1st, 2013 when their name changed because of the unethical way they were originally named for a Nazi Scientist who used the dead corpses of murdered victims who had been executed by the Third Reich. Max Clara  used the tissue from these victims for research that led to his discovery of the Clara cells which were named after him in 1937 from his research at Leipzig.