Explain how the structure of the trachea and bronchi allows them to carry out their functions in the respiratory system.

The trachea and bronchi are kept open by C‑shaped rings of hyaline cartilage, which prevent collapse during breathing and allow air to flow freely into the lungs.
The inner lining contains ciliated columnar epithelium and goblet cells that secrete mucus. The mucus traps dust, microbes and other particles, and the cilia beat rhythmically to transport the mucus upward towards the pharynx, where it can be swallowed or expelled, keeping the lower airways clear.