Left main bronchus

At a glance

English

Left main bronchus

Latin (TA2)

Bronchus principalis sinister

System

Respiratory

Region

Thorax

Side

Single, on the midline

About the left main bronchus

The left main bronchus begins at the carina, the ridge at the lower end of the trachea, roughly opposite the sternal angle, at the disc between the fourth and fifth thoracic vertebrae. It is about five centimetres long, more than twice the length of the right main bronchus, and has a smaller calibre. It also leaves the trachea at a wider angle, running more obliquely to the left, because it has to pass under the arch of the aorta and clear the heart to reach the left lung.

On its way it runs below the aortic arch, crossing in front of the oesophagus, thoracic duct and descending aorta. The left pulmonary artery lies first above it and then in front. It enters the hilum of the left lung at about the level of the sixth thoracic vertebra. There it divides into two lobar bronchi, superior and inferior, one for each lobe of the left lung. Unlike the right bronchus it gives off no branch above its pulmonary artery, so every left airway is hyparterial.

Its wall is built like that of the trachea. Incomplete C-shaped rings of hyaline cartilage hold it open, their open backs bridged by smooth muscle and fibrous tissue. Inside, it is lined by ciliated respiratory epithelium with mucus-secreting goblet cells and seromucous glands, which trap dust and move it upwards towards the throat. No gas exchange takes place here; the bronchus only conducts air.

Because of its steeper angle and narrower bore, inhaled foreign bodies and over-advanced breathing tubes enter the left bronchus less often than the right. Its close relation to the aortic arch means that an aneurysm of the arch, or a greatly enlarged left atrium below, can compress it. Inflammation of the bronchi produces bronchitis, and in asthma the smooth muscle constricts and the lining swells.

Function and supply

Begins at

The bifurcation of the trachea at the carina, roughly opposite the sternal angle.

Ends at

Enters the left lung at the hilum and divides into the superior and inferior lobar bronchi.

Function

Conducts air between the trachea and the left lung, warming, moistening and cleaning it on the way.

Nerve supply

Pulmonary plexus: vagal fibres that narrow the airway and increase secretion, and sympathetic fibres that widen it.

Blood supply

Bronchial arteries, usually two on the left, arising from the descending thoracic aorta; bronchial veins drain to the accessory hemi-azygos system and pulmonary veins.