The left main bronchus (or left primary bronchus, or left principal bronchus) is smaller in caliber but longer than the right, being 5 cm long. It enters the root of the left lung opposite the sixth thoracic vertebra.
It passes beneath the aortic arch, crosses in front of the esophagus, the thoracic duct, and the descending aorta, and has the left pulmonary artery lying at first above, and then in front of it.
The left bronchus has no eparterial branch, and therefore it has been supposed by some that there is no upper lobe to the left lung, but that the so-called upper lobe corresponds to the middle lobe of the right lung.
Additional images 
The tracheobronchial lymph glands.
The position and relation of the esophagus in the cervical region and in the posterior mediastinum. Seen from behind. it is red.
External links 
This article incorporates text from a public domain edition of Gray's Anatomy.