Final answer:
Cartilage, which supports the trachea and bronchi, becomes less dense in the bronchioles and is completely absent in the smallest airways, ensuring proper air delivery to the alveoli for gas exchange.
Step-by-step explanation:
The cartilage within the respiratory system, which is most abundant in the trachea and bronchi, becomes much less dense in the bronchioles and is eventually absent. The trachea and bronchi are supported by C-shaped rings of hyaline cartilage which prevent their collapse. As air moves from the trachea into the bronchi, and finally into the smaller bronchioles, the structure changes significantly. The bronchioles lack cartilage altogether and rely on the inhalation of air to maintain their shape. This transition from cartilaginous to non-cartilaginous passageways is important in the respiratory system, allowing for efficient air delivery to the alveoli where gas exchange occurs.