What is the function of trachea? Why do the walls not collapse even when there is less air in it?

Trachea is a tube that connects pharynx with lungs for carrying air to and from lungs. Trachea is lined by ciliated mucus secreting pseudostratified epithelium for trapping dust particles and microbes.

Trachea does not collapse in reduced air pressure due to support of C-shaped cartilaginous rings.