WHAT ARE LUNG SEGMENTS? - ANATOMY AND PHYSIOLOGY - 2025

The lung segments are portions or divisions of the lungs with specific features and functions. This division occurs thanks to the bronchial bifurcation.

The pulmonary segments can also be considered macroscopic units of the lung, provided with a bronchus (segmental bronchus) and a complementary branch of the pulmonary artery.

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Each of these segments has between 30 and 60 bronchioles, a particular venous and arterial supply, so they function independently.

They cannot be seen from the outside of the lung, but doctors must use radiography or bronchoscopy, and are separated by connective tissue septa.

The images obtained to detect these segments in the lung are usually incomplete or confusing.

What are lung segments for?

The lung segments serve to guide thoracic surgeons in interpreting images of the lungs and in surgical procedures.

The right lung is divided into 3 lobes (upper, middle and lower), divided with 2 fissures, while the left has 2 lobes with a fissure.

For the interpretation of two-dimensional images, scientists created a classification system of vessels and bronchi in 5 lobe regions, by detecting edge and curved surface, taking advantage of the linear appearance of fissures in the lobes.

In addition, there is a kind of global lung atlas that serves as a template and is coded. Regarding three-dimensional images, the methods use the Gaussian approach and the analysis of Hessian matrices.

It is worth saying that a weakness of these systems based on anatomical knowledge is that they ignore individual variability and could therefore lead to segmentation failures when the methods are applied to the "new" exams.

Whereas if the focus is based on shapes in the image space, these risks of errors decrease.

Lung segmentation is normally done based on the location and direction of the main and segmental bronchi.

Description of the lung segments

1- Right segments

In the right lung there are 10 segments:

• Apical (S1): It is V-shaped and constitutes the apex of the lung that extends to the second rib.
• Posterior (S2): It is shaped like a quadrilateral directed backwards and whose thoracic wall is related to the second, third and fourth ribs.
• Anterior (S3): It is located parallel to the bronchi of the middle lobe, from the hilum to the periphery. It is almost quadrilateral in shape and is divided into two sub-segments.
• Lateral (S4): it is located in the middle lobe of the lung and is observed on the costal aspect. With its triangular shape, it forms a kind of wedge between horizontal and oblique fissures.
• Medial (S5): it is a segment that is best seen on the mediastinal side, from which it can be seen occupying the entire surface of the middle lobe of the lung, just in the median part of the horizontal fissure with which it already fuses near the heart.
• Superior (S6): This segment originates from the posterior aspect of the main bronchus, it has a triangular shape with vertebrae T4 to T8 at its base.
• Medial basal (S7): it is located below the pulmonary hilum and is one of those that appears on the diaphragmatic face like all segments from 6 to 10. It is also called the cardiac segment and constitutes an important part of the mediastinal surface.
• Basal anterior (S8): It is the segment located in the ventral section of the oblique fissure of the lung.
• Lateral basal (S9): This segment also has the shape of a triangle whose vertex points to the hilum and whose base occupies the costophrenic angle.
• Basal posterior (S10): Another triangular segment. In this case, the vertex points to the hilum and the base - a middle two-thirds of the diaphragm.

2- Left segments

In the left lung there are also 10 segments, but since there are cases in which they are ventilated through the same bronchus, two are counted as one.

It is also worth clarifying that there are a couple of marked differences with respect to the segments of the right lung: the combination of segments and the presence of a "lingular lobe" incorporated into the upper lobe of the left lung, replacing the middle lobe.

• Apic and posterior (S1 and S2) Although they could be considered as two segments, the reality is that they are seen as one only because they are ventilated by the same bronchus.
• Previous (S3): is located horizontally.
• Lingular inferior (S4): This segment and the one that follows (S5), are related by the lingula of the left lung.
• Lingular superior (S5)
• Superior (S6)
• Antero-medial basal (S7 and S8)
• Lateral basal (S9)
• Posterior basal (S10)

It should be noted that there is a nomenclature to name the elements of each lung segment (segmental bronchus, artery and veins).

The rules of this nomenclature dictate that a capital letter must be used (S, B, A or V, depending on whether it refers to segments, lobar bronchi, arteries or vein), followed by a number, which indicates to which segment the element belongs in question.

That is, the name of the element already illuminates its location in the lung. For example: B2 refers to the bronchus that ventilates segment 2.

Segmental bronchi

They are the bronchi that carry air to the lung segments.

Right segmental bronchi

The B1, B2 and B3 are those that ventilate the segments that make up the upper lobe of the lung, while B4 and B5 ventilate the middle lobe.

The lower lobe of the right lung is ventilated by bronchi B6 to B10

This lobe is the area with the most bronchi, because it is also the area with the most volume and lung parenchyma, so it is the part that needs more ventilation.

Left segmental bronchi

In the case of the bronchi that ventilate the left lung, from B1 to B5 they go to the upper lobe; B7 and B8 ventilate the anteromedial basal segment and B6 through B10 go to the lower lobe segments.

References

1. Cure Cancer (2017). The bronchopulmonary segments of the lungs in Clinical features. Recovered from clicktocurecancer.info.
2. Hennings Hinojosa, Érika & others (2002). Normal anatomy of the airway and lung segmentation by helical computed tomography in Anales de radiología México 2002; 4 587-589. Recovered from medigraphic.com.
3. National Center for Biotechnology Information (s / f). Segments of lungs. Recovered from ncbi.nlm.nih.gov
4. San Martín, Gustavo (s / f). Lung segmentation. Recovered from authorstream.com.