Final answer:
Smooth muscle fibers in the bronchus walls help regulate air flow by controlling the diameter of the bronchioles through bronchoconstriction and bronchodilation.
Step-by-step explanation:
The smooth muscle fibers in the bronchus walls have an important function in regulating air flow in the lungs. They help to control the diameter of the bronchioles, which are the smaller passages that air passes through in the lungs. When these smooth muscles contract, it leads to bronchoconstriction, which decreases the size of the bronchioles. On the other hand, when these smooth muscles relax, it leads to bronchodilation, which increases the size of the bronchioles, allowing for more air to flow in and out of the lungs.