The bronchi and bronchioles are where air passes through either the nose or mouth, and toward the alveoli in the lungs. The airway splits into left and right braches. It splits down further and further until it reaches the terminal bronchioles. The bronchi have a very similar structure to the trachea. They both have c-shaped cartilage. There are no cartilage structure on the bronchioles.
The main function of the bronchi and bronchioles is to carry oxygen from the trachea into the lungs. Smooth muscle is located within the walls to help control airflow into the lungs.
At the end of each bronchiole is alveoli. Alveoli are tiny sacs which allow oxygen and carbon dioxide to move between the lungs and bloodstream.