Posterior segmental artery of left lung

The posterior segmental artery of the left lung (A2) is a branch of the left pulmonary artery that supplies the posterior segment (S2) of the left upper lobe. However, a key anatomical distinction of the left lung is that, unlike the right lung where the apical (A1) and posterior (A2) segmental arteries typically arise as separate vessels, in the left lung the apical and posterior segments are fused into a single apicoposterior segment (S1+2), and their arterial supply correspondingly merges into a common segmental artery designated A1+2.

Anatomical characteristics:

• Origin: The left A1+2 arises directly from the left main pulmonary artery, which is a direct posterior continuation of the main pulmonary trunk. Unlike the right side, the left pulmonary artery does not divide into distinct superior and inferior trunks but rather runs in the interlobar fissure with a variable branching pattern.

• Territory: It supplies the apicoposterior segment (S1+2) of the left upper lobe, which occupies the apex and posterior aspect of the upper lobe. This fused segment corresponds to the separate apical (S1) and posterior (S2) segments found in the right upper lobe.

• Subsegmental branches: The A1+2 typically divides into subsegmental branches designated A1+2a, A1+2b, and A1+2c. CT studies have shown that A1+2c runs along the posterior aspect of its corresponding bronchus (B1+2c) in approximately 82% of cases. The most common branching pattern according to the Yamashita classification is Type A, in which the left upper lobe receives A3, A1+2a+b, and A1+2c as separate trunks.

• Variability: The arterial anatomy of the left upper lobe exhibits the greatest variability in branching patterns among all pulmonary lobes. The left upper lobe is most commonly supplied by four arterial branches from the left pulmonary artery, though three and five branches are also frequently observed. The A1+2 itself has at least four recognized subtypes of branching. Differences in arterial branching patterns between Asian and Western populations have also been noted.