General anatomy of the lungs

Lobar structure and main fissures

Each lung is divided into lobes surrounded by visceral pleura. The left lung consists of two lobes (superior and inferior), separated by the main oblique interlobar fissure (fissura obliqua). The right lung has three lobes: superior, middle, and inferior. They are separated by two fissures — the oblique and the additional horizontal (fissura horizontalis).

It is important to note that interlobar fissures are often incomplete, especially in the medial regions. This anatomical feature creates alveolar pathways for collateral air drift and also promotes the unobstructed spread of diseases between adjacent lobes.

Anatomy of the main (oblique) fissures

The oblique fissures are symmetrical on both sides. They begin approximately at the level of the fifth thoracic vertebra (Th5), descend obliquely forward and downward, pass through the lung root (hilum), and reach the diaphragm. The point of contact with the diaphragm is located 0–3 cm posterior to the anterior costophrenic sinus.

Spatially, the oblique fissure is a smoothly curved plane that can be compared to a propeller blade: its upper half is turned anteriorly and laterally, and its lower half anteriorly and medially. The lower part of the fissure sometimes widens due to adipose tissue accumulation or pleural thickening. This increases the contact area with the diaphragm and causes a local loss of silhouette definition — the so-called "juxtaphrenic peak" sign (juxtaphrenic peak).

Horizontal fissure and imaging visualization

The horizontal fissure is present only on the right. It fans out horizontally from the right root toward the chest wall, contacting it at the level of the sixth rib in the midaxillary line. The anterior and lateral parts of the fissure curve smoothly downward.

For the fissure to be visible on a standard radiograph, the X-ray beam must pass strictly tangentially to its plane.

• In the posteroanterior (PA) projection, part of the horizontal fissure can be noticed in most patients, whereas the oblique fissures are usually obscured.
• In the lateral projection, fragments of both fissures are often visible. Due to the undulating course of the oblique fissure, it may project as two lines on a lateral view, which is sometimes mistakenly interpreted as its displacement.

On computed tomography (MSCT), normal fissures are visualized well. If the fissure line itself is indistinct, the relatively avascular zone of the outer cortex of the lobe serves as a landmark. On axial slices, the horizontal fissure does not form a clear line (it is defined as an oval zone of reduced vascularity at the level of the bronchus intermedius), therefore multiplanar reconstruction (MPR) is required for its evaluation.

Accessory fissures and pulmonary ligaments

In addition to the main ones, accessory interlobar fissures exist. The most well-known is the azygos lobe fissure (azygos lobe fissure), found in 1% of people. It is formed due to impaired migration of the azygos vein, which invaginates the layers of the visceral and parietal pleura. The term "lobe" here is incorrect, as the segmental structure of the lung does not change. On CT, an atypical course of the vein is visible in this zone, and the "lobe" itself may appear less radiopaque.

Other variants of accessory fissures:

1. A fissure on the left, separating the lingual segments from the rest of the superior lobe (an analogue of the right horizontal fissure).
2. The superior accessory fissure, horizontally separating the apical segment from the basal segments in the inferior lobes.
3. The inferior accessory fissure (more commonly on the right), running obliquely upward from the diaphragm to the root and separating the medial basal segment from the anterior one.

Also, an important structure is the inferior pulmonary ligaments (ligamenta pulmonalia inferiora). These are pleural reflections hanging from the lung roots to the diaphragm (analogous to peritoneal ligaments). Directly beneath them within the lung, the intersegmental septum of the inferior lobe is often visualized.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Topography of segments of the superior, middle, and inferior lobes of the lungs
• CT anatomy of the lung root and bronchial tree
• Mechanisms of collateral air drift through incomplete interlobar fissures
• Heart and respiratory motion artifacts during HRCT
• Pathological pleural changes: effusions and thickenings along the fissures

Other topics in the «Chest Radiology» section

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