Inferior lingular segment of left lung

The inferior lingular segment (S5) of the left lung is one of the two bronchopulmonary segments that comprise the lingular division of the left upper lobe. The lingular division is unique to the left lung and is considered the left-sided homologue of the right middle lobe.

Anatomical definition: The inferior lingular segment is defined by its segmental bronchus (B5), its corresponding segmental artery (A5), and its draining vein (V5). The lingular division bronchus typically bifurcates into the superior lingular segmental bronchus (B4) and the inferior lingular segmental bronchus (B5), with this classic B4B5 pattern present in approximately 96.5% of individuals. The inferior lingular segment lies below and posterior to the superior lingular segment (S4), extending along the anteroinferior and medial surface of the left lung, bordered by the cardiac notch medially and the oblique (major) fissure posteriorly.

Key features:

• Bronchus: B5 (inferior lingular segmental bronchus), arising from the lingular division bronchus (B4+5).

• Arterial supply: The inferior lingular artery (A5) most commonly arises from the interlobar pulmonary artery. In approximately 24% of cases, A5 may originate from A8 or A8+9, and in about 8% of cases its origin is close to A8 or A8+9, reflecting significant anatomical variability.

• Venous drainage: V5 drains into the superior pulmonary vein and is described as the least variable of the lingular segmental veins.

• Boundaries: The inferior lingular segment is separated from the superior lingular segment (S4) superiorly, and an accessory fissure between S4 and S5 is identifiable on CT in approximately 17% of cases. The left minor fissure, when present, separates the lingula from the anterior segment (S3) above.