Final answer:
The diameter of bronchioles can change due to the smooth muscle in their walls allowing for bronchoconstriction and bronchodilation.
Step-by-step explanation:
The diameter of bronchioles can change because their walls contain smooth muscle.
The walls of bronchioles are composed of smooth muscle and elastic fibers, which facilitate the adjustment in diameter of the bronchioles. This muscular arrangement makes it possible for the bronchioles to either constrict or dilate, known respectively as bronchoconstriction and bronchodilation, controlling airflow to the lungs. Bronchi and bronchioles are part of the respiratory system, where air is channeled from the mouth and nose down through the trachea and into smaller and smaller passageways until it reaches the terminal bronchioles and then the alveoli for gas exchange. Unlike the larger airways that are reinforced with cartilage, such as the trachea and main bronchi, the smaller bronchioles lack cartilage, relying instead on their muscle fibers to support their walls. The sympathetic nervous system generally induces bronchodilation, increasing air passage capacity, while the parasympathetic nervous system stimulates bronchoconstriction, reducing airflow when necessary.