A bronchiole is one of the tiny, thin-walled, tubular branches of the bronchi. There are three types of bronchioles: lobular, terminal, and respiratory one. Lobular bronchioles arise from a small bronchus, and from each lobular bronchiole four to five terminal bronchioles originates. Several respiratory bronchioles stem from a terminal one; the walls of these latter bronchioles are formed by a cuboidal epithelium. In turn, each respiratory bronchiole divides into several alveolar ducts, which end up in thin-walled sacs called alveoli, which are clustered together like a bunch of grapes.