Bronchus

A bronchus (plural bronchi, adjective bronchial), also known as a main or primary bronchus, is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles. No gas exchange takes place in the 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 or up to two vertebrae higher or lower, depending on breathing, at the anatomical point the carina of trachea.

The right main bronchus is wider, shorter, and more vertical than the left main bronchus. It enters the right lung at 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 it from behind; and the right pulmonary artery lies at first below and then in front of it.

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