Lateral segment of right lung

The lateral segment of the right lung (S4) is one of the two bronchopulmonary segments of the right middle lobe. It is defined as the discrete, functionally independent unit of lung parenchyma that is ventilated by the lateral segmental bronchus (B4) and supplied by the corresponding branch of the pulmonary artery, with venous drainage via the intersegmental veins.

Anatomical characteristics:

• Location: The lateral segment occupies the outer (costal/lateral) portion of the right middle lobe, facing the chest wall. It is bounded superiorly by the horizontal (minor) fissure separating it from the right upper lobe, inferiorly by the oblique (major) fissure separating it from the right lower lobe, and medially by the intersegmental plane shared with the medial segment (S5).

• Bronchovascular supply: The right middle lobe bronchus most commonly bifurcates into two segmental bronchi B4 (lateral) and B5 (medial) in approximately 91% of individuals. In about 89% of cases, a trifurcation pattern (B4, B5, and an accessory bronchus B) is observed. The segmental artery closely accompanies B4, and within the middle lobe, the medial subsegmental bronchus of the lateral segmental bronchus is characteristically located medial to its corresponding pulmonary artery a relationship that is opposite to that seen in the right upper lobe and can be used on CT to differentiate the two lobes.

• Surface projection: On chest imaging, the lateral segment projects onto the anterolateral aspect of the right hemithorax, roughly at the level of the 4th to 6th ribs.