The trachea is a long tube that remains constantly open to allow the passage of air. It is about 10-12 cm long and extends from the larynx at the level of the sixth cervical vertebra to the level of the fourth or fifth thoracic vertebra, where it divides into the two bronchi. This division is called the tracheal bifurcation.

The mucous membrane of the trachea is continuous with that of the larynx above and the bronchi below. It rests on a strong elastic layer and contains many mucous glands, especially in the membranous wall and near the annular ligaments. The epithelium is ciliated and helps move mucus and particles upward through the airway.

The trachea receives blood mainly from the inferior thyroid arteries. Its nerves come from the vagus nerves, recurrent laryngeal nerves, and the sympathetic nervous system.

Bronchi

The trachea divides at the carina into the right and left primary bronchi. The carina contains sensitive nervous tissue that triggers coughing if foreign material enters the airway. 

• The bronchi are lined by pseudostratified ciliated columnar epithelium with goblet cells, which trap dust and pathogens in mucus.

The bronchi enter the lungs through the hilum and continue branching to form the bronchial tree. Their main function is to conduct air and help clean it before it reaches the lungs. 

The right bronchus is shorter, wider, and more vertical than the left, so inhaled objects are more likely to enter it. It is about 2.5 cm long and gives off an upper branch called the eparterial bronchus before continuing as the hyparterial bronchus to supply the middle and lower lobes of the right lung.