Principal bronchus (left)

The left principal bronchus forms the extrapulmonary division of the trachea that supplies the left lung. It starts at the tracheal bifurcation at the level of the sternal angle. It ends within the parenchyma of the left lung hilus where it divides into the two left lobar bronchi:

• superior lobe bronchus
• inferior lobe bronchus This differs from the right side where there is a division of the principal bronchus before the hilum and three lobar bronchi.

The left principal bronchus is longer than the right side, typically about 5cm in length. It also runs more horizontally from the tracheal bifurcation and is of smaller cross-section. This makes the left side less susceptible to the entry of foreign bodies which descend the trachea.

The relations of the left principal bronchus include:

• superiorly: pulmonary arterial trunk and left pulmonary artery branch
• anteroinferiorly: pulmonary veins
• posteriorly: thoracic aorta

The relations of the left principal bronchus are described further in the section on the structure of the left lung root.