Brad H. Thompson, M.D., William J. Lee, B.S., Jeffrey R.
Galvin, M.D. and Jeffrey S. Wilson, M.D
Peer Review Status: Internally Reviewed
Left Upper Lobe
(S1+2) represents a combination of the two separate (apical and posterior) segments seen on the right upper lobe. It is referred to as the apical posterior segment. Combined, S1+2 anatomically is similar in shape to the two segments on the right. Likewise, the S3 or anterior segment also is similar to the right S3 segment, having a large area directed anteriorly.
On the left there is no middle lobe; the anatomical equivalent region corresponding to the right middle lobe is known as the lingula, and like the RML, is also composed of two segments. Unlike their counterparts on the right however, the segments are stacked one on top of another, rather than side by side. The superior lingular segment (S4) is positioned immediately inferior to S3 and is directly above the inferior lingular segment (S5). Both lingular segments have their greatest area directed anteriorly and inferiorly. Additionally, S4 tends to have more area directed laterally while S5 has more area directed medially, abutting the mediastinum. Both segments extend anteroinferiorly to reach the ventral surface of the left hemidiaphragm, and hence border a large portion of the anteroinferior surface of the major fissure.
Left Lower Lobe
The left lower lobe segmental architecture is similar to that of the right lower lobe, however because the anterior and medial basal segments share a common bronchial supply, these two segments are characteristically combined, forming an anterior medial basal segment (S8). Therefore, the anterior segment is combined with the medial segment but may be denoted by SX7. The anterior medial basal segment is bordered anteriorly by the interlobar (major fissure), and abuts the visceral pleural surfaces of the two lingular segments located anterosuperiorly. Forming the remainder of the interlobar surface is the superior segment (S6). S6 is identical to its counterpart on the right.
The lateral basal segment, S9 and the posterior basal segment, S10 also closely approximate the same segments in the right lower lobe. S9 (lateral basal segment) has its broadest surface area exposed posterolaterally, and may be slightly larger than S9 on the right. S9, not in continuity with the major fissure, occupies much of the posterior costophrenic sulcus posterolaterally. Like its contralateral counterpart, S10 (posterior basal segment) is quite large and occupies the posterior and inferior most portion of the right lower lobe occupying the posteromedial costophrenic gutter.
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