Bronchi are the respiratory passageways between the trachea and the lobes of the lung. After the left and right primary bronchi (also termed principal bronchi) arise from the trachea, they decrease in diameter as they ramify (create multiple extensions) and enter the lobes of the lung as lobar and segmental bronchi. The multiple branching of the bronchi is referred to as a bronchial tree.

Corresponding to the lobes of the lung they serve, the right principal bronchus divides into a right superior lobe broncus, a middle lobe bronchus, and right inferior lobe bronchus. The left principal bronchus divides into the left superior principal bronchus and the left inferior lobe bronchus.

In the embryo, the bronchi develop from the caudal end of the tracheal tube as primordial main stem bronchi. As ultimately reflected in adult anatomy, the left main bronchus assumes a more transverse course than the right main...