Primary bronchi

The trachea (windpipe) divides into two main or primary bronchi, the left and the right, at the level of the sternal angle and of the fifth thoracic vertebra. The bifurcation may be located as high as the third thoracic vertebra or as low as the seventh thoracic vertebra, depending on breathing. The carina is the cartilage that demarcates the bifurcation of the trachea. 

The right main bronchus (thick green arrow in the figure below) is wider, shorter, and more vertical than the left main bronchus. It enters the right lung at the level of (approximately) the fifth thoracic vertebra. The right main bronchus subdivides into three secondary bronchi (also known as lobar bronchi), which deliver air to the three lobes of the right lung. The azygos vein arches over the right main bronchus from behind. The right pulmonary artery lies at first below and then in front of the right main bronchus.

The left main bronchus (thin green arrow in the figure below) is smaller in caliber but longer than the right, being 5 cm long. It enters the root of the left lung at the level of the sixth thoracic vertebra. The left main bronchus divides into two secondary or lobar bronchi to deliver air to the two lobes of the left lung. The left main bronchus 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.