The left main bronchus is smaller in caliber but longer than the right

Right main bronchus

The right main bronchus is a bronchus of the respiratory system. It is wider, shorter, and more vertical in direction than the left...

, being nearly 5 cm long. It enters the root of the left lung

Left lung

The human left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilum....

 opposite the sixth thoracic vertebra.

It passes beneath the aortic arch

Aortic arch

The arch of the aorta or the transverse aortic arch is the part of the aorta that begins at the level of the upper border of the second sternocostal articulation of the right side, and runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left...

, crosses in front of the esophagus

Esophagus

The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, the thoracic duct

Thoracic duct

In human anatomy, the thoracic duct of the lymphatic system is the largest lymphatic vessel in the body. It is also known as the left lymphatic duct, alimentary duct, chyliferous duct, and Van Hoorne's canal....

, and the descending aorta

Descending aorta

The descending aorta is part of the aorta, the largest artery in the body. The descending aorta is the part of the aorta beginning at the aortic arch that runs down through the chest and abdomen. The descending aorta is divided into two portions, the thoracic and abdominal, in correspondence with...

, and has the left pulmonary artery

Pulmonary artery

The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

 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

Right lung

The human right lung is divided into three lobes , superior, middle, and inferior, by two interlobular fissures:-Fissures:...

.