Lateral basal segment of right lung

The lateral basal segment of the right lung (S9) is one of the five bronchopulmonary segments of the right lower lobe. It is a discrete, functionally and anatomically independent unit of lung parenchyma, each bronchopulmonary segment being defined by its own segmental bronchus (B9), segmental artery (A9), and drained by intersegmental veins at its periphery.

Anatomical characteristics:

• Location: S9 occupies the lateral aspect of the base of the right lower lobe, situated between the anterior basal segment (S8) anteriorly and the posterior basal segment (S10) posteriorly. Its lateral surface abuts the costal (chest wall) pleura, and its base rests on the diaphragmatic pleura.

• Bronchus: It is ventilated by the lateral basal segmental bronchus (B9), which arises from the basal trunk of the right lower lobe bronchus. In the most common branching pattern, the right inferior lobe bronchial tree divides into B6 (superior segment), B7 (medial basal), B8 (anterior basal), B9 (lateral basal), and B10 (posterior basal). Anatomic variations are common in some individuals B9 may share a common trunk with B8 or B10, and in approximately 18% of cases B9 is absent as a distinct bronchus, with its territory supplied by adjacent segments.

• Vasculature: The segmental artery (A9) accompanies B9 centrally within the segment, while the segmental veins run along the intersegmental planes at the periphery, draining into the inferior pulmonary vein. The arteries generally lie toward the lung periphery relative to their corresponding bronchi, while the veins lie more centrally (medially).

• Boundaries: The intersegmental planes separating S9 from adjacent segments (S6 superiorly, S8 anteriorly, S10 posteriorly, and S7 medially) are defined by the intersegmental veins and connective tissue septa, though these planes are not visible on gross inspection and are identified surgically or radiologically.