Middle lobe of right lung

The middle lobe of the right lung (lobus medius pulmonis dextri) is the smallest of the three lobes of the right lung, situated in the anteroinferior portion of the right hemithorax. It is unique to the right lung, as the left lung has no true middle lobe - the lingula of the left upper lobe is considered its anatomical counterpart.

Boundaries and Fissures

The middle lobe is demarcated by two fissures:

• Superiorly, the horizontal (minor) fissure separates it from the right upper lobe.

• Posteroinferiorly, the oblique (major) fissure separates it from the right lower lobe.

The horizontal fissure is complete in approximately 54% of individuals, incomplete in 35%, and absent in about 11%. The oblique fissure on the right is complete in roughly 77% of cases.

Bronchopulmonary Segments

The middle lobe contains two bronchopulmonary segments, each supplied by its own segmental bronchus, artery, and vein:

• Lateral segment (B4) located laterally

• Medial segment (B5) located medially, abutting the mediastinum

The middle lobe bronchus arises from the bronchus intermedius (the continuation of the right main bronchus after the takeoff of the right upper lobe bronchus). In the most common branching pattern (~91% of individuals), the middle lobe bronchus bifurcates into B4 and B5. A trifurcation variant (B4, B5, and an accessory branch B) occurs in approximately 8.6% of cases.

A distinguishing CT feature is that within the middle lobe, the subsegmental bronchi are located medial to their corresponding pulmonary arteries the opposite relationship to that seen in the right upper lobe, where subsegmental bronchi lie lateral to their arteries. This consistent relationship can be used to differentiate the two lobes on crosssectional imaging.