Structure and Characteristics
The left main bronchus begins at the tracheal bifurcation (bifurcatio tracheae, carina tracheae). The level of division corresponds to the intervertebral disc between the 4th and 5th thoracic vertebrae posteriorly (projecting to the 5th–6th vertebrae) and the sternal angle or the second intercostal space anteriorly.
Compared to the right bronchus, the left bronchus is narrower and longer. Its average diameter is 1.2 cm. The angle of divergence from the trachea is approximately 120°, making its course less vertical than that of the right main bronchus.
The wall contains 9–12 cartilaginous half-rings (cartilagines tracheales), a posterior membranous wall (paries membranaceus), and smooth muscle fibers (m. trachealis). The inner surface is lined by a mucous membrane (tunica mucosa) featuring pseudostratified ciliated columnar epithelium and mucous glands (glandulae tracheales, bronchiales).
Topography and Neighboring Structures
The bronchus runs obliquely downwards and laterally toward the hilum of the left lung, which is located inferior to the tracheal bifurcation.
- Aortic arch (arcus aortae) — passes superior to the left main bronchus and arches over the root of the left lung from anterior to posterior.
- Esophagus (esophagus) — lies posterior to the trachea; inferior to the aortic arch, it relates to the posterior surface of the left main bronchus. These organs are connected by connective tissue bands known as the tracheoesophageal ligaments.
- Left recurrent laryngeal nerve (n. laryngeus recurrens sinister) — travels in the tracheoesophageal groove between the esophagus and the trachea.
- Fascia and fatty tissue — the trachea, bronchi, and esophagus are enclosed by a common tracheoesophageal fascial sheath, which delimits the interbronchial space. Regional lymph nodes are located in this area: bifurcation, tracheobronchial, and paratracheal nodes.
Syntopy in the Root of the Left Lung
At the hilum of the left lung, the root forms with elements arranged in the following spatial organization:
- Pulmonary artery — occupies the most superior position. It arches over the left main bronchus and then runs along the posterolateral surface of the left inferior lobar bronchus.
- Left main bronchus — lies inferior and posterior to the pulmonary artery.
- Pulmonary veins — the superior and inferior pulmonary veins are 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.
Branches of the Left Main Bronchus
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 branching of the superior lobar bronchus. It gives off the segmental bronchus to the superior segment of the lower lobe (VI), and then continues as the basal bronchus, which divides into segments VII (inconstant), VIII, IX, and X.