Muscle tissue found in the walls of airways that can contract or relax, influencing bronchial diameter and airflow.

• A. Inflammation
• B. Smooth muscle
• C. Beta-agonists
• D. Chemoreceptors

Answer: (B)

Airway diameter is controlled by smooth muscle in the walls of the bronchi and bronchioles. This muscle can contract to narrow the airways (bronchoconstriction) or relax to widen them (bronchodilation), directly changing airway resistance and airflow. Autonomic signals regulate this: sympathetic stimulation via beta-2 receptors promotes relaxation and increased airflow, which is why beta-agonist medications are used to relieve bronchospasm. Inflammation can narrow airways through swelling and mucus, but it’s not the tissue type that changes diameter. Chemoreceptors adjust breathing in response to blood gas levels, not the smooth muscle’s diameter.