The trachea is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus. This mucus lines the cells of the trachea to trap inhaled foreign particles that the cilia then waft upward toward the larynx and then the pharynx where it can be either swallowed into the stomach or expelled as phlegm.

image from          Mouseover for labeled slide
Notice the “C” shaped rings of the hyaline cartilage.  These support the tracheal mucosa and prevent its collapse during inspiration.
Bands of smooth muscle called the trachealis muscle join the ends of the rings posteriorly.  When they contract, they reduce the diameter of the trachea and help by increasing the intrathoracic pressure during coughing.
Wheater, Functional Histology

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