The tiny (all less than 1 millimeter in diameter), branching airways that lead from the bronchi to the alveoli Bronchioles have elastic fibers and smooth muscle but (unlike bronchi) no cartilage Most bronchioles also produce mucus
Small airway passages that branch off the lungs' larger bronchial tubes At the end of the bronchiole are the alveole, which are little sac-like structures where 'dirty' blood gets re-oxygenated before routing back through the heart If the air we inhale cannot make it through the bronchiole to the alveole, the blood's oxygen saturation level will go down Here's an illustration of bronchiole during an asthma flare