What is the main characteristic of the trachea?

The main characteristic of the trachea being composed of 16-20 cartilaginous rings is significant because these rings provide essential structural support to maintain the open airway necessary for proper ventilation. The rings are C-shaped, which allows flexibility and expansion during breathing while preventing the collapse of the trachea. This arrangement ensures that air can flow freely from the larynx to the bronchi and into the lungs.

The cartilaginous rings are crucial as they keep the trachea rigid, which helps protect against obstruction from external pressures or internal changes, such as during swallowing. The presence of these rings also distinguishes the trachea from other structures in the respiratory system, which may be made of different types of tissue that can be more prone to collapse without such support.

Understanding the structural composition of the trachea is essential for comprehending its function in the respiratory system. The other options do not accurately represent the trachea's anatomy or function—smooth muscle is present but not the primary structural component, the trachea does not branch directly into alveoli (it branches into bronchi), and it indeed has structural support rather than lacking it.