Segmental bronchus

A segmental bronchus (Tertiary bronchus) is a third-generation airway that arises from a lobar (secondary) bronchus and supplies a specific bronchopulmonary segment of the lung. In the standard airway hierarchy, the trachea (generation 0) divides into the two main (primary) bronchi, which divide into lobar (secondary) bronchi serving each lobe, which then divide into segmental (tertiary) bronchi each ventilating an anatomically and functionally distinct bronchopulmonary segment.

There are typically 10 segmental bronchi on the right (corresponding to 10 bronchopulmonary segments) and 8-10 on the left (some segments share a common bronchus, particularly in the left upper lobe).

Like other conducting airways, segmental bronchi have walls composed of mucous membrane, smooth muscle, and cartilaginous support.

Each segmental bronchus is accompanied by a corresponding segmental pulmonary artery (running together in a bronchovascular bundle), while the segmental pulmonary veins typically run in the intersegmental planes between adjacent segments.

The internationally accepted naming system was established in 1949 by an ad hoc committee at the International Congress of Otorhinolaryngology and later codified in the Nomina Anatomica (1955). Boyden's nomenclature, based on postmortem specimens, remains widely used to describe segmental bronchial anatomy.

The two main bronchi (right an left) give 5 lobar bronchi (for the 5 pulmonary lobes) that then divide into 20 segmental bronchi (for the 20 segments of the lung):

• Right superior lobar bronchus

  • Apical segmental bronchus [B I]

  • Posterior segmental bronchus [B II]

  • Anterior segmental bronchus [B III]

• Middle lobar bronchus

  • Lateral segmental bronchus [B IV]

  • Medial segmental bronchus [B V]

• Right inferior lobar bronchus

  • Intermediate bronchus (term not include in Terminologia Anatomica but widely used)

  • Superior segmental bronchus [B VI]

  • Medial basal segmental bronchus B VII

  • Anterior basal segmental bronchus [B VIII]

  • Lateral basal segmental bronchus [B IX]

  • Posterior basal segmental bronchus [B X]

• Left superior lobar bronchus

  • Apicoposterior segmental bronchus  [B I + II]

  • Anterior segmental bronchus [B III]

  • Superior lingular bronchus [B IV]

  • Inferior lingular bronchus [B V]

• Left inferior lobar bronchus

  • Superior segmental bronchus [B VI]

  • Medial basal segmental bronchus [B VII]

  • Anterior basal segmental bronchus [B VIII]

  • Lateral basal segmental bronchus [B IX]

  • Posterior basal segmental bronchus [B X]

Branching patterns show considerable variation. In the left upper lobe, for example, the most common pattern is a common stem of the apical and posterior segmental bronchi (~64%), followed by trifurcation (~23%). In the lower lobes, variant arterial and bronchial patterns occur in up to 20% of cases.