Structure and characteristics
The left main bronchus begins at the region of the tracheal bifurcation (bifurcatio tracheae, carina tracheae). The level of division corresponds to the intervertebral disc between the IV and V thoracic vertebrae posteriorly (projection onto the spine is the V–VI vertebrae) and the sternal angle or the second intercostal space anteriorly.
Compared to the right, the left bronchus is narrower and longer. Its average diameter is 1.2 cm. The angle of divergence from the trachea is approximately 120°, which makes its course less vertical than that of the right bronchus.
The wall contains 9–12 cartilaginous semirings (cartilagines tracheales), a posterior membranous part (paries membranaceus), and smooth muscle fibers (m. trachealis). The inner surface is lined with a mucous membrane (tunica mucosa) containing pseudostratified ciliated epithelium and mucous glands (glandulae tracheales, bronchiales).
Topography and adjacent structures
The bronchus runs obliquely downwards and laterally toward the hilum of the left lung, which is located inferior to the level of the tracheal bifurcation.
- Aortic arch (arcus aortae) — passes over the left main bronchus and arches over the root of the left lung from anterior to posterior.
- Esophagus (esophagus) — located posterior to the trachea; inferior to the aortic arch, it lies adjacent to the posterior surface of the left main bronchus. The organs are connected by connective tissue strands — the tracheoesophageal ligaments.
- Left recurrent laryngeal nerve (n. laryngeus recurrens sinister) — runs in the tracheoesophageal groove between the esophagus and the trachea.
- Fascia and adipose tissue — the trachea, bronchi, and esophagus are surrounded by a common tracheoesophageal fascial sheath, which delimits the interbronchial space. Regional lymph nodes are located in this zone: bifurcation, tracheobronchial, and paratracheal lymph nodes.
Syntopy in the root of the left lung
At the hilum of the left lung, the root is formed, the elements of which have the following spatial organization:
- Pulmonary artery — occupies the uppermost position. It arches over the left main bronchus superiorly, then runs along the posterolateral surface of the left inferior lobar bronchus.
- Left main bronchus — is located inferior and posterior to the pulmonary artery.
- Pulmonary veins — the superior and inferior pulmonary veins lie adjacent to the anterior and inferior surfaces of the bronchus and artery.
Posterior to the root of the left lung on the medial surface lies the aortic groove.
What branches does the left main bronchus divide into?
At the hilum of the left lung, the left main bronchus divides into two lobar bronchi:
- Superior lobar bronchus — divides into two trunks. The superior trunk includes segments I–II (apicoposterior) and III (anterior). The inferior (lingual) trunk includes segments IV and V.
- Inferior lobar bronchus — originates inferior to the origin of the superior lobar bronchus. It gives off the segmental bronchus to the apical segment of the lower lobe (VI), and then continues as the basal bronchus, which divides into segments VII (inconstant), VIII, IX, and X.
Class and exam questions
How does the left main bronchus differ from the right?
The left bronchus is longer, narrower (average diameter 1.2 cm vs. 2 cm for the right), and diverges from the trachea at a more acute angle (~120°). Its wall contains more cartilaginous semirings (9–12 vs. 6–8 on the right).
How are the elements arranged in the root of the left lung?
In the root of the left lung, the pulmonary artery occupies the highest position. The left main bronchus lies inferior and posterior to it, and the pulmonary veins lie adjacent to the anterior and inferior surfaces of the bronchus and artery.
Which major vessel arches over the left main bronchus?
The aortic arch passes over the left main bronchus and arches over the root of the left lung from anterior to posterior.
What else to study on this topic
- Tracheal bifurcation
- Root of the left lung
- Segments of the left lung
- Aortic arch
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