The small tubes forming the airways that travel deep into the lungs. They extend from the two bronchi (branches of the windpipe) and they end in small air sacs called alveoli.
The tiny branches of air tubes within the lungs which are the continuation of bronchi and connect to the alveoli (air sacs). ... (12 Dec 1998) ... 
(Humans as organisms) the many small, branching tubules into which the bronchi subdivide
The bronchioles are the intermediate air passages within the lungs. They branch off of the large bronchi and extend to the smaller branches of the alveolar ducts. Each respiratory bronchiole subdivides into five or more alveolar ducts. The structure of the bronchi, bronchioles, alveolar ducts, and alveoli is often called the ' pulmonary tree' becau...
Smaller branches of air passages leading from the bronchi to the alveoli (the small air sacs within the lungs, through the walls of which gas exchange between the blood and air occurs).
smaller air passages within the lung that are formed as the bronchi become further subdivided
(bronchiole) These are the smallest airways in the lungs. They connect the smallest tertiary bronchi to the alveoli (air sacs) where oxygen is absorbed.
Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli.