Parentheses are different in different parts. Except that there are bones near the nasal cavity, they are basically cartilage.

The first picture shows the structure of each cartilage in the larynx.

The second picture shows the distribution structure of the whole trachea.

Thyroid cartilage is ring-shaped, with open trachea and bronchial cartilage, type C.

At the bottom of the respiratory tract is alveoli, where there is no cartilage as a scaffold. Alveolus depends on two forces to maintain its open state. Surfactant is the main force to maintain surface tension. The other is the negative pressure in pleural cavity.