The bronchioles or bronchioli are the first passageways by which the air passes through the nose or mouth to the air sacs of the lungs in which branches no longer contain cartilage or glands in their submucosa. They are branches of the bronchi. The bronchioles terminate by entering the circular sacs called alveoli.Alveoli have thin walls. The exchange of gases between the blood and the air takes place through these walls. A pulmonary lobule is the portion of the lung ventilated by one bronchiole. Bronchioles are approximately 1mm or less in diameter and their walls consist of ciliated cuboidal epithelium and a layer of smooth muscle. Bronchioles divide into even smaller terminal bronchioles that are 0.5mm or less in diameter. Terminal bronchioles in turn divide into respiratory bronchioles which divide into alveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in the respiratory system while respiratory bronchioles are the beginning of the respiratory division where actual gas exchange takes place. The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called bronchodilation and is stimulated by either epinephrine or sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction and is stimulated by Histamine, parasympathetic nerves, cold air, chemical irritants and other factors to decrease air flow.