Final answer:
The trachea bifurcates into the mainstem bronchi at the level of T4 to T5, forming a bronchial tree that functions as a passageway for air and traps debris and pathogens.
Step-by-step explanation:
The trachea bifurcates into the right and left mainstem bronchi at the level of the carina, which is located at the spinal level of T4 to T5. The primary bronchi then enter the lungs at the hilum and further branch into the secondary bronchi, which continue to divide into tertiary bronchi, bronchioles, and eventually to the terminal bronchioles. The structure of the bronchi is supported by rings of cartilage that prevent their collapse, and they are lined by pseudostratified ciliated columnar epithelium that contains mucus-producing goblet cells. This system forms the bronchial tree, which functions as a passageway for air to move into and out of the lungs, as well as trapping debris and pathogens.