How does smooth muscle in the bronchial walls change airway diameter?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

The smooth muscle located within the bronchial walls plays a critical role in regulating the respiratory system by controlling the diameter and length of the airways.

When these smooth muscle layers contract, they narrow the bronchial passages, which can obstruct airflow. This mechanism is particularly significant in conditions like asthma, where inflammation and increased sensitivity cause the muscle to contract excessively. Conversely, the relaxation of these muscles allows the airways to widen, facilitating the movement of air. This muscular activity is essential for the proper regulation of airflow during the expiration of air from the lungs.1