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Lung segments are a high-yield topic tested in anatomy, propedeutics (segmental auscultation), radiology (chest X-ray interpretation), and surgery (segmentectomy). Memorizing them as a list of 20 names is useless: you need to understand the logic of division and visual maps of their locations. Below is a table, principles, and projections.
What is a bronchopulmonary segment
A segment is a portion of the lung ventilated by a segmental bronchus (tertiary bronchus) and supplied by a corresponding branch of the pulmonary artery; it has a pyramidal shape with the apex pointing toward the lung root and the base facing the surface. Segments are separated from each other by connective tissue septa containing intersegmental veins. This makes the segment an anatomical unit that can be resected independently, as well as the primary site for focal pathological processes.
Segment table
| № | Right lung | Proportion | Left lung | Proportion |
|---|---|---|---|---|
| S1 | Apical (apicalis) | Upper | Apicoposterior, S1+2 | Upper |
| S2 | Posterior (posterior) | |||
| S3 | Anterior (anterior) | Anterior (anterior) | ||
| S4 | Lateral (lateralis) | Mean | Superior lingular (lingularis superior) | |
| S5 | Medial (medialis) | Inferior lingular (lingularis inferior) | ||
| S6 | Superior (superior) | Inferior | Superior (superior) | Inferior |
| S7 | Medial basal (basalis medialis) | Medial basal — often absent or fused with S8 | ||
| S8 | Anterior basal (basalis anterior) | Anterior basal | ||
| S9 | Lateral basal (basalis lateralis) | Lateral basal | ||
| S10 | Posterior basal (basalis posterior) | Posterior basal |
Total: the right lung has 10 segments, and the left has 8–10 (usually considered 9: S1+2 are fused, S7 is typically absent). The numbering is identical on both sides, which is the most important thing to memorize first: S6 is always the superior segment of the lower lobe, and S10 is the posterior basal.
Differences between the right and left lungs
- The right lung has three lobes (upper, middle, lower) and two fissures (oblique and horizontal); the left lung has two lobes and one oblique fissure.
- The middle lobe of the right lung corresponds to the lingula in the left lung, which is a part of the upper lobe containing segments S4 and S5.
- In the left lung, S1 and S2 share a common bronchus — the S1+2 segment.
- The right main bronchus is shorter, wider, and more vertical, which is why foreign bodies and aspirates more frequently enter the right side.
- The left lung is narrower and longer due to the cardiac notch.
How to memorize
Upper lobe: apical, posterior, anterior (1–2–3, from top to bottom and back to front). Middle lobe / lingula: lateral, medial on the right and superior, inferior lingular on the left (4–5). Lower lobe: superior (6) and four basal segments arranged in a circle: medial (7), anterior (8), lateral (9), posterior (10) — clockwise when looking at the right lung from below. Mnemonic for basal segments: clockwise order.
Projection of segments on a radiograph
In the anteroposterior (AP) view, segments overlap one another, and it is impossible to accurately localize a lesion on a single plain radiograph — a lateral view is required. Landmarks:
| Zone on PA radiograph | Projected segments |
|---|---|
| Apex above the clavicle | S1 (S1+2 on the left) |
| Subclavicular zone | S2 (posterior) and S3 (anterior) — overlapping |
| Hilar zone | S6 (posterior), S3, S4–5 |
| Lower lung field near the heart | S5, S7, S8 (anterior) |
| Lower lung field laterally | S4, S9 |
| Above the diaphragm near the spine | S10 (posterior) |
On the lateral radiograph, the oblique fissure runs from the level of Th3–4 posteriorly down and forward to the diaphragm; everything posterior to it is the lower lobe (S6 superiorly, S10 inferiorly near the spine, S8 near the anterior diaphragm). The horizontal fissure on the right separates the middle lobe at the level of the 4th rib anteriorly. A lesion in the upper lobe on a lateral view lies anterior to the oblique fissure, while a lower lobe lesion lies posterior. Read more in the article Chest X-ray: interpretation algorithm.
Clinical significance: typical localizations
- Tuberculosis — S1, S2, S6 (apical and posterior segments: poorly ventilated, well aerated with oxygen, and have weaker lymphatic drainage).
- Aspiration pneumonia — in supine patients: S2 and S6 (posterior); in standing/sitting patients: S8–S10 (basal), more often on the right.
- Community-acquired pneumonia — more frequently lower lobes, S9–S10; in children — S4–S5 and S6.
- Middle lobe syndrome — atelectasis of S4–S5 due to a long, narrow middle lobe bronchus surrounded by lymph nodes.
- Pulmonary infarction in PE — subpleurally in the basal segments of the lower lobes.
- Peripheral cancer — more often S3 and S6; central cancer — segmental and lobar bronchi of the upper lobes.
Segments and auscultation
Anteriorly above the clavicle S1 is auscultated, below the clavicle — S3, in the axillary region — S4–S5 on the right and the lingula on the left, S9; posteriorly above the scapula — S1–S2, interscapularly — S6 (at the level of the scapular angle), below the scapula — S10, along the axillary line — S9. Knowing this, the localization of rales allows naming the segment before a chest X-ray. Diagrams of auscultation points are available in the propaedeutics guide.
Frequently asked questions
How many segments are in the left lung — 8, 9, or 10? According to the International Anatomical Terminology — 10, of which S1 and S2 share a common bronchus, and S7 is inconstant. In Russian textbooks, "9–10" is more common. On the exam, a safe answer is: "10 segments, with S1 and S2 combined into S1+2, and S7 often absent."
Which segment is the largest? S10 (posterior basal) and S6 — in the lower lobes.
Why is a segment, rather than a lobe, the unit of description? Because focal processes are limited to the segment by connective tissue septa, and a segment can be resected in isolation.
See also: Pneumonia: Diagnosis and Treatment · Tuberculosis · Pneumothorax · Anatomy Reference