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Radiology · Chest Radiology

Lung segments on X-ray

Segmenta bronchopulmonalia

On standard chest radiographs, the projections of bronchopulmonary segments significantly overlap one another. Understanding their complex three-dimensional topography is critical for accurate localization of pathological processes, although due to ventilation features, they rarely have sharp boundaries on images.

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SuperpositionPulmonary segment shadows heavily overlap in the anteroposterior projection
No septaThere are no anatomical septa between segments (with rare exceptions)
End-on projectionBronchi running along the X-ray beam produce ring-shaped shadows on images
Difference on the leftThe seventh (medial basal) segment is absent in the left lung
3 min readFor medical studentsUpdated 10.10.2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesPending instructor review

Features of segmental X-ray anatomy

Bronchopulmonary segments form in exact accordance with the branching of the bronchial tree. The boundaries between these areas have a very complex three-dimensional shape, comparable to a 3D puzzle.

Standard posteroanterior and lateral radiographs show pronounced overlap (overlap) of segment projections onto each other. This superposition is much more pronounced in the anteroposterior projection.

It is important to remember that normally there are no septa between segments (with rare exceptions). Because of this, so-called collateral air drift occurs across segmental boundaries. For this reason, pathological processes such as atelectasis or focal pneumonia, while potentially predominating within a specific segment, rarely strictly adhere to its anatomical boundaries.

The walls of the segmental bronchi themselves are usually not visible on a standard radiograph. An exception is when a bronchus is projected strictly orthogonally (end-on projection)—then ring shadows (ring shadows) are visualized on the image. In the absence of concomitant interstitial edema, this pattern is entirely within normal limits.

Right lung segment nomenclature

According to the British Thoracic Society nomenclature, the right lung has 10 segments distributed across three lobes.

Upper lobe:

  • 1. Apical bronchus (Apical bronchus)
  • 2. Posterior bronchus (Posterior bronchus)
  • 3. Anterior bronchus (Anterior bronchus)

Middle lobe:

  • 4. Lateral bronchus (Lateral bronchus)
  • 5. Medial bronchus (Medial bronchus)

Lower lobe:

  • 6. Apical bronchus (Apical bronchus)
  • 7. Medial basal / cardiac bronchus (Medial basal / cardiac bronchus)
  • 8. Anterior basal bronchus (Anterior basal bronchus)
  • 9. Lateral basal bronchus (Lateral basal bronchus)
  • 10. Posterior basal bronchus (Posterior basal bronchus)
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Left lung segment nomenclature

The anatomy of the left lung differs somewhat: instead of a middle lobe, there is the lingula (Lingula), and the total number of segments is reduced due to the absence of the seventh bronchus.

Upper lobe:

  • 1. Apical bronchus (Apical bronchus)
  • 2. Posterior bronchus (Posterior bronchus)
  • 3. Anterior bronchus (Anterior bronchus)

Lingula:

  • 4. Superior bronchus (Superior bronchus)
  • 5. Inferior bronchus (Inferior bronchus)

Lower lobe:

  • 6. Apical bronchus (Apical bronchus)
  • 8. Anterior basal bronchus (Anterior basal bronchus)
  • 9. Lateral basal bronchus (Lateral basal bronchus)
  • 10. Posterior basal bronchus (Posterior basal bronchus)
How to memorize

To easily remember the absence of the 7th segment in the left lung, recall its name: "medial basal / cardiac". On the left, this space is occupied by the heart itself, so no separate cardiac segment is distinguished there.

Class and exam questions

Why is pneumonia rarely confined to exactly one segment?

There are no dense anatomical septa between lung segments. Air and pathological exudate can move between adjacent areas across boundaries via collateral drift.

What are normal ring shadows?

Ring shadows (ring shadows) represent the orthogonal projection of bronchi when the beam runs parallel to their lumen. In the absence of interstitial edema, this is a normal radiological finding.

Can the walls of all bronchi be seen on a standard chest radiograph?

No. Segmental bronchial walls are not visualized on planar radiographs, except for cases of their orthogonal projection. Further branches (bronchioles) are also invisible due to their small caliber and lack of cartilage.

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