Left main bronchus

The left main bronchus is the major air-conducting tube that branches from the trachea at the tracheal bifurcation, channeling air to the left lung.

The left main bronchus is one of the two main bronchi, emerging from the trachea at the carina. It extends caudally and laterally to the left to enter the left lung at its hilum. Compared to the right main bronchus, it is generally longer, narrower, and its angle of divergence from the trachea is often more acute. Its wall consists of cartilaginous rings or plates, smooth muscle, and a ciliated respiratory mucosa. It then branches into lobar bronchi for the lobes of the left lung.

The anatomy of the left main bronchus is relatively more constant across species than that of the right, as there is never a tracheal bronchus branching off the trachea for the left lung.

In dog, cat, Ruminants and horse, the left main bronchus originates from the trachea at the carina. It then proceeds towards the left lung and branches into lobar bronchi for the cranial and caudal lobes of the left lung.

In humans, the left main bronchus is generally longer, narrower, and more horizontal than the right main bronchus. It branches off the trachea at the carina. As in domestic animals, there is no tracheal bronchus for the left lung. The left main bronchus gives rise to the lobar bronchi for the two lobes of the left lung (superior and inferior). Its more horizontal trajectory makes it less susceptible to foreign body aspiration compared to the right main bronchus.