Architectonics of the bronchial tree
The main bronchi have distinct anatomical differences: the left main bronchus (bronchus principalis sinister) is significantly longer than the right (bronchus principalis dexter). Segmental bronchi undergo 6 to 20 divisions, transitioning into smaller branches—bronchioles (bronchioles).
The last structures with an exclusively conducting function are the terminal bronchioles (terminal bronchioles). Beyond them, the respiratory zones begin. The branching of segmental and subsegmental bronchi exhibits high variability.
Segmental structure and nomenclature
Bronchopulmonary segments are formed based on bronchial branching. The boundaries between them have a complex three-dimensional shape. With rare exceptions, dense septa are absent between segments, which allows for collateral air drift.
On axial CT slices, bilateral intersegmental septa (intersegmental bilateral septa) can be identified. On the right, such a septum may have a bifurcated T-shape that clearly demarcates segments 9 and 10.
Classification according to the nomenclature of the Thoracic Society:
- Right lung:
- Upper lobe: 1 — Apical, 2 — Posterior, 3 — Anterior.
- Middle lobe: 4 — Lateral, 5 — Medial.
- Lower lobe: 6 — Apical, 7 — Medial basal (cardiac), 8 — Anterior basal, 9 — Lateral basal, 10 — Posterior basal.
- Left lung:
- Upper lobe: 1 — Apical, 2 — Posterior, 3 — Anterior.
- Lingula: 4 — Superior, 5 — Inferior.
- Lower lobe: 6 — Apical, 8 — Anterior basal, 9 — Lateral basal, 10 — Posterior basal. (Note: the 7th segment is absent on the left in this nomenclature).
The lower lobe is fixed to the paraoesophageal mediastinum (paraoesophageal mediastinum) by the pulmonary ligament (ligamentum pulmonale inferius). This ligament is clearly visible in pneumothorax: the lung tissue collapses, but the lower lobe remains attached to the mediastinum.
Lobules, acini, and the lymphatic system
The basic gas-exchange structure is the acinus (acinus). It includes respiratory bronchioles, alveolar ducts, and alveoli. Acini group together to form the secondary pulmonary lobule (secondary pulmonary lobule) at the periphery of the lung.
Secondary lobules are separated by peripheral interlobular septa. Subpleural and deep lymphatic vessels run within them, draining lymph to the lung root. Normally, they are not visible on radiographs. When these septa thicken, septal lines appear on images:
- Kerley B lines (Kerley B lines) — due to thickening of the interlobular septa.
- Kerley A lines (Kerley A lines) — due to thickening of the deep peribronchial septa.
Imaging features
On chest radiographs, lung structures overlap each other (overlap).
- Bronchi: Bronchial walls are normally not visible unless they are in orthograde projection (parallel to the radiation beam). In this case, they produce ring shadows (ring shadows).
- Vessels: In the outer 2/3 of the lungs, it is impossible to distinguish arteries from veins. In the upright position (standing), vessel caliber increases from the apices to the bases (physiological perfusion gradient). In the supine position, this difference disappears. Normally, the diameter of vessels in the first intercostal space does not exceed 3 mm, and the ratio of the arterial diameter to the adjacent bronchus is 1:1.
- Density: On a lateral radiograph, the density of the spine shadow should gradually decrease from top to bottom. There should be no abrupt density changes over the cardiac shadow (except for epicardial fat pads and rib shadows).
Briefly: answers to topical questions
What is the anatomical difference between the left main bronchus and the right?
The left main bronchus (bronchus principalis sinister) differs from the right (bronchus principalis dexter) in its greater length, smaller diameter, and angle of origin from the trachea.
| Sign | Right main bronchus | Left main bronchus |
|---|---|---|
| Length | Shorter | Almost twice as long |
| Width (diameter) | Wider (average 2 cm) | Narrower (average 1.2 cm) |
| Angle of origin | Blunter (direct continuation of the trachea) | More acute |
| Cartilaginous semirings | 6–8 | 9–12 |
| Topography | More vertical course | Passes under the aortic arch (arcus aortae) |
Which vessels run in the center of the secondary pulmonary lobule, and which ones run in the interlobular septa?
Centrilobular pulmonary arteries (or arterioles) run in the center of the secondary pulmonary lobule; they accompany the lobular bronchiole and serve as a marker of the lobular center. The pulmonary veins, along with their accompanying deep lymphatic vessels, are located in the peripheral interlobular septa (peripheral interlobular septa) that separate the secondary lobules.
To which regional lymph nodes does lymph from the lungs drain?
Lymph from the lungs and pleura drains into the lymph nodes of the lung root and tracheal region. Regional lymph nodes include:
- Intrapulmonary nodes (nodus lymphoideus intrapulmonalis) — located within the lung parenchyma.
- Bronchopulmonary nodes (nodi lymphoidei bronchopulmonales) — located in the hila of the lungs.
- Tracheobronchial nodes (nodi lymphoidei tracheobronchiales) — include superior (superiores) and inferior bifurcation (inferiores) nodes.
Drainage occurs via deep lymphatic pathways running peribronchially and in the deep septa of the lungs.
Through which microscopic anatomical structures does collateral air drift occur in the lungs?
Collateral air drift in the lungs occurs through the pores of Kohn (pores of Kohn). These are interalveolar fenestrations located in the interalveolar septa. They connect the spaces of various neighboring alveolar units, ensuring the equalization of air pressure between them and maintaining lung expansion during proximal airway obstruction.
The absence of the 7th segment in the left lung (medial basal) is easy to remember: its anatomical space is "occupied" by the heart.
Class and exam questions
Why do pneumonia or atelectasis rarely have strict boundaries corresponding to a single segment?
Dense septa are practically absent between pulmonary segments. This causes collateral air drift across segmental boundaries, causing the pathological process to spread to adjacent areas.
How to distinguish bronchioles from bronchi anatomically?
The main morphological difference of bronchioles is the absence of cartilage plates in their walls. They form after 6–20 divisions of the segmental bronchi.
When are the bronchial walls normally visible on a radiograph?
Only when the bronchus is projected strictly in orthograde projection (end-on). In this case, it forms a ring shadow on the image.
What else to study on this topic
- Volumetric CT technique with thin collimation for airway evaluation.
- Assessment of the symmetry of lung field density on direct radiographs.
- Topography of pulmonary veins and arteries in the region of the upper and lower lobe hila.
- Intrapulmonary lymph nodes: CT findings and differential diagnosis.
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