Superior lobe of left lung

The superior lobe of the left lung (Left upper lobe; LUL) is one of the two lobes of the left lung, separated from the inferior lobe by the left oblique (major) fissure. Unlike the right lung, which has three lobes, the left lung has only two due to the space occupied by the cardiac notch.

Segmental Anatomy

The left upper lobe is divided into two major anatomical divisions:

• Upper division (culmen or trisegment): Contains the fused apicoposterior segment (S1+2) and the anterior segment (S3). The apicoposterior segment is a fusion of what would be separate apical (S1) and posterior (S2) segments on the right side, reflecting the smaller volume of the left upper thoracic cavity during embryologic development.

• Lingular division: Contains the superior lingular segment (S4) and the inferior lingular segment (S5). The lingula is considered the anatomical counterpart of the right middle lobe.

This two-division structure is functionally analogous to the right upper lobe plus the right middle lobe.

Boundaries

The left oblique fissure separates the upper lobe from the lower lobe. On surface anatomy, the left oblique fissure most commonly originates posteriorly at the level of the fourth rib (45% of cases), crosses the midaxillary line at the sixth rib (52%), and reaches the anterior hemidiaphragm inferiorly; its most frequent shape is linear (78%). The left oblique fissure is complete in approximately 72% of individuals, incomplete in 26%, and absent in about 2%. Accessory fissures within the left upper lobe (e.g., the left minor fissure separating the anterior segment from the superior lingular segment) are present in approximately 8% of left lungs.

Bronchovascular Anatomy

The left upper lobe bronchus arises from the left main bronchus and divides into the superior division bronchus and the lingular division bronchus. The most common bronchial branching pattern of the superior division is a common stem of the apical and posterior segmental bronchi (B1+2) with a separate anterior segmental bronchus (B3), seen in approximately 76% of individuals. The arterial supply arises from the left pulmonary artery, most commonly via four branches, with the most frequent pattern being Type A in the Yamashita classification (A3, A1+2a+b, A1+2c). Venous drainage is via the left superior pulmonary vein, though lingular vein drainage into the left inferior pulmonary vein occurs in approximately 4% of cases.