Principles of segmental structure
The formation of bronchopulmonary segments occurs through the branching of segmental bronchi. It is important to understand that the boundaries between segments have a complex three-dimensional configuration (resembling a 3D puzzle), with virtually no septa (septation) between them.
Due to this anatomical fact, pathological processes such as atelectasis or pneumonia, although they may predominate within a single segment, rarely strictly adhere to its geometric boundaries. This is facilitated by collateral air drift across segmental junctions.
On plain chest radiographs, the walls of the segmental bronchi are usually not visible. An exception is when a bronchus is projected strictly along the path of the X-ray beam (orthograde projection, end-on projection). In this case, it appears as a ring shadow (ring shadow), which in the absence of interstitial edema is an absolute normal variant.
Nomenclature of pulmonary segments
A standardized nomenclature of bronchial division is used in diagnostic radiology. According to the classification of the British Thoracic Society, the structure is as follows:
Right lung:
- Upper lobe: 1 - Apical, 2 - Posterior, 3 - Anterior.
- Middle lobe: 4 - Lateral, 5 - Medial.
- Lower lobe: 6 - Apical (superior), 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 left segmental bronchus is absent in this nomenclature).
Hila and pulmonary vessels
On radiographs and CT scans, the density of normal lung hila is primarily determined by blood vessels. The bronchial walls are thin and contribute only marginally to the hilar shadow. Normal lymph nodes are not identified on conventional images, but are clearly visible on modern multislice CT scans.
An important landmark on the anteroposterior radiograph is the hilar points, which are the intersection points of the superior pulmonary vein and the descending lower lobe artery. Normally, the left hilar point is always located at the same level as the right one or slightly higher.
Regarding the pulmonary vasculature, in the outer 2/3 of the lungs it is impossible to distinguish arteries from veins. However, certain patterns exist in the central zones:
- In the lower lobes, the veins run more horizontally, while the arteries run vertically.
- In the upper lobes, the vessels have a similar smooth vertical curve, but the veins lie lateral to the arteries.
- In the upright position, vessel diameters naturally increase from the apexes to the bases due to the perfusion gradient. In the supine position, this difference is leveled out.
Microstructure: from bronchioles to the secondary lobule
Segmental bronchi divide repeatedly (up to 20 times), transitioning into bronchioles (bronchioles), whose main distinguishing feature is the absence of cartilage in their walls. The terminal bronchioles are the last structures to perform a purely air-conducting function.
They are followed by gas-exchange structures forming the acinus (acinus) with a diameter of 5–6 mm:
- Respiratory bronchioles
- Alveolar ducts
- Alveoli
A group of 3–5 acini forms the secondary pulmonary lobule (secondary pulmonary lobule). At the periphery of the lung, these lobules are bounded by interlobular septa (interlobular septa). Normally, these septa and the lymphatic vessels running within them are not visible. However, when they thicken (due to edema, cells, or fibrosis), linear shadows appear on the images—Kerley B lines (Kerley B lines), while changes in the deep septa produce Kerley A lines.
To memorize the topography of the hila on an AP radiograph: "The left lung lies on the heart, therefore its hilum (left point) is elevated higher than the right one."
Class and exam questions
Why does pneumonia not always have clear borders along the segmental contour?
There are no rigid anatomical septa between the lung segments. Due to collateral air drift, infection and exudate can freely cross segment boundaries.
How to assess pulmonary artery dilation on CT or X-ray?
Normally, the diameter of the artery and the adjacent bronchus (evaluated in an end-on projection) are approximately equal—about 4–5 mm. If the artery is wider than the bronchus by more than 1.5 times, this is a sign of its dilation.
Are normal hilar lymph nodes visible?
On a standard plain radiograph, normal lymph nodes are not visible. However, on multislice CT, they are frequently visualized as small, ellipsoidal peripheral nodules.
What else to study on this topic
- HRCT patterns of diffuse lung diseases
- Causes of interlobular septal thickening (Kerley lines)
- "Honeycomb lung" and traction bronchiectasis on CT
- MRI semiotics of normal hilar structures
- Physiological perfusion gradient and its effect on vessel caliber
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