Medial segment of right lung

The medial segment of the right lung (S5) is one of the two bronchopulmonary segments of the right middle lobe. It is the portion of the right middle lobe that lies adjacent to the mediastinum and the right cardiac border. It is bounded superiorly by the minor (horizontal) fissure, which separates it from the right upper lobe, and posteroinferiorly by the major (oblique) fissure, which separates it from the right lower lobe. Laterally, it borders the lateral segment (S4) of the right middle lobe, from which it is separated by an intersegmental plane.

Key anatomical features:

• Bronchus: It is supplied by the medial segmental bronchus (B5), which arises from the middle lobe bronchus. The middle lobe bronchus typically bifurcates into the lateral (B4) and medial (B5) segmental bronchi, though trifurcation variants have been described in approximately 8.6% of individuals.

• Vascular supply: The medial segmental pulmonary artery accompanies the medial segmental bronchus. A distinguishing feature on CT is that the subsegmental bronchi of the medial segment (superior and inferior subsegmental bronchi) are located medial to their corresponding pulmonary arteries a relationship opposite to that seen in the right upper lobe, which can be used to differentiate the two lobes on crosssectional imaging.

• Surface relations: The medial segment faces the mediastinum and the right heart border, giving it clinical relevance in conditions such as middle lobe syndrome, where recurrent atelectasis or infection preferentially affects this region.