Medial basal segment of left lung

The medial basal segment of the left lung (S7) is one of the bronchopulmonary segments of the left lower lobe. It is defined as the parenchymal territory ventilated by the medial basal segmental bronchus (B7) and supplied by the corresponding branch of the pulmonary artery (A7). The segment is situated on the medial (mediastinal) surface of the left lower lobe, resting against the mediastinum and the diaphragm, and is bordered by the anterior basal segment (S8) laterally and anteriorly, the lateral basal segment (S9) posterolaterally, and the superior segment (S6) superiorly.

Nomenclature and context within the left lower lobe

Using the internationally accepted Boyden nomenclature (standardized in 1949 by the ad hoc committee of the International Congress of Otorhinolaryngology and subsequently adopted in the Nomina Anatomica), the left lower lobe is divided into the following segments:

• S6 Superior segment

• S7 Medial basal segment

• S8 Anterior basal segment

• S9 Lateral basal segment

• S10 Posterior basal segment

On the right side, S7 is a constant and well-recognized segment. On the left side, however, S7 has historically been considered frequently absent or rudimentary, leading many textbooks and older surgical references to describe only four basal segments (S8-S10 plus S6) in the left lower lobe.

Prevalence and anatomical variability

A large retrospective study using 3D-CT bronchography and angiography in 1,440 patients found that the left medial basal bronchus (B7) is present in approximately 75% of individuals, with six distinct branching patterns identified. The most common pattern (61.4%) is B7 arising from B8; in 24.4% of cases, B7 was entirely absent. Other less common origins include B7 arising independently above the B8B10 trunk (6.3%), from B9 (5.5%), or from combined origins (1).