Asthma. Artwork of a lung bronchiole that has been sectioned to show the mucus (yellow) that blocks the airflow in asthma. Mucus is produced because of bronchiole constriction caused by triggers such as allergens. The ring of smooth muscle (red, seen in the bronchiole wall) contracts, prompting mucus production by cells in the epithelial layer lining the inside of the bronchiole. Rings of cartilage (purple) support the bronchiole, which is a small tube formed by branching and narrowing of the main airway from the mouth. A bronchiole ends in sacs of alveoli (one seen at top left) where oxygen and carbon dioxide is exchanged with blood vessels (blue and red). Nerves (yellow) are also seen.