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Lobar Anatomy

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 Peer Reviewed

A brief review of the lobar architecture of both lungs is necessary to better understand the bronchial and segmental anatomy presented later in detail.

Right Lung
There are three lobes in the right lung: 1) right upper lobe (RUL), (2) right middle lobe (RML), and (3) right lower lobe (RLL). The right upper lobe (RUL) occupies the upper 1/3 of the right lung. Posteriorly, the RUL is adjacent to the first three to five ribs. Anteriorly, the RUL extends inferiorly as far as the 4th right anterior rib. Xray of lungs Xray of lungs The right middle lobe is typically the smallest of the three, and appears triangular in shape, being narrowest near the hilum. Xray of lungs Xray of lungs The right lower lobe (RLL) Xray of lungs Xray of lungs is the largest of all three lobes, separated from the others by the major fissure. Xray of lungs Posteriorly, the RLL extend as far superiorly as the 6th thoracic vertebral body, and extends inferiorly to the diaphragm. Review of the lateral plain film surprisingly shows the superior extent of the RLL; there is considerable overlap between the more anterosuperiorly located RUL and the RLL. Xray of lungs Similarly, the deep posterior gutters extend considerably inferiorly; with full inspiration, the lower lobe can extend may as low as L2, becoming superimposed over the upper poles of the kidneys.

Grossly, these lobes can be separated from one another by two fissures which anatomically correspond to the visceral pleural surfaces of those lobes from which they are formed. The minor fissure separates the RUL from the RML, and thus represents the visceral pleural surfaces of both of these lobes. Xray of lungs The minor fissure is oriented horizontally, extending ventrally from the chest wall, and extending posteriorly to meet the major fissure. Generally, the location of the minor fissure is approximately at the level of the fourth vertebral body and crosses the right sixth rib in the midaxillary line. The right major fissure is more expansive in size than the minor fissure, separating the right upper and middle lobes from the larger right lower lobe. Xray of lungs Oriented obliquely, the major fissure extends posteriorly and superiorly approximately to the level of the fourth vertebral body. The major fissure extends anteroinferiorly, intersecting the diaphragm at the anterior cardiophrenic angle.

Left Lung
The lobar architecture of the left lung is slightly different than the right. Because there is no defined left minor fissure, there are only two lobes on the left; left upper Xray of lungsXray of lungs and left lower lobes. Xray of lungsXray of lungs These two lobes are separated by a major fissure, identical to that seen on the right side, although often slightly more inferior in location. Xray of lungs The portion of the left lung that corresponds anatomically to the right middle lobe is incorporated into the left upper lobe. Xray of lungsXray of lungs

It is important to understand that in most individuals, interlobar fissures are usually not completely formed; in some individuals there may be complete absence of a fissure thus losing the demarcation between lobes on gross examination. Review of autopsy materials have revealed that complete development of the minor fissure is seen less than 20% of the time. Similarly, complete development of the right major fissure is seen in less than 30% of the population. Conversely, approximately 1% of individuals have complete absence of an interlobar fissure. Furthermore, at the level of the hilum (or pulmonary root) the pulmonary lobes are not routinely separated from one another, again due to incomplete development of the interlobar fissures medially.

In general, fissures are not readily identifiable on plain films, with only small portions typically visualized at best. This is because fissures which are composed of only two layers of visceral pleura, may not present a significant radiographic interface and will not produce a shadow. However, if there is fluid within the pleural space or if the visceral pleura is thickened, fissures may be seen in their entirety.

Neither the major or minor fissures are definitively demonstrated on CT. In fact, because of the axial orientation of the right minor fissure, exact delineation of the border between the right middle and upper lobes is almost impossible on CT. The approximate locations of the major fissures are inferred from areas of relatively sparse pulmonary vascularity. While separation of pulmonary lobes solely on the basis of fissure location remains rather difficult even on CT, fissural anatomy is not helpful at all in the identification of bronchopulmonary segments. For these reasons, an understanding of bronchial anatomy is the easiest and most reliable way to identify individual pulmonary segments.

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