Left Lung Segments

The left lung is divided into a series of anatomically organized bronchopulmonary segments. Each segment is a pyramidal or wedge-shaped region of pulmonary tissue supplied by a segmental, or tertiary, bronchus and an accompanying segmental branch of the pulmonary artery.

Bronchopulmonary segments are functionally and surgically important subdivisions of the lung. Although the exact number and naming of segments can vary because adjacent segments may share bronchi or be described as combined units, the left lung is commonly organized into approximately 8 to 10 bronchopulmonary segments.

Organization of the Left Lung

The left lung contains two lobes:

• Superior lobe
• Inferior lobe

The lobes are separated primarily by the oblique fissure. Unlike the right lung, the left lung does not normally have a horizontal fissure or a separate middle lobe.

Bronchopulmonary Segments

A bronchopulmonary segment is a subdivision of a pulmonary lobe centered on a segmental bronchus and its accompanying pulmonary arterial branch.

The segments are separated from neighboring segments by connective tissue planes that are incomplete but sufficiently distinct to permit anatomical and surgical identification.

Basic Structure of a Segment

Each bronchopulmonary segment is approximately pyramidal in shape. Its apex is directed toward the lung root, while its broader base faces the pleural surface.

The segmental bronchus and pulmonary arterial branch generally occupy a relatively central position within the segment, whereas pulmonary veins tend to run in the connective tissue planes between adjacent segments.

Left Superior Lobe

The superior lobe of the left lung contains the upper division and the lingula. The lingula is a tongue-shaped projection of the superior lobe that lies adjacent to the left border of the heart.

The segmental arrangement of the left superior lobe differs from that of the right lung because some segmental bronchi commonly arise from shared trunks.

Apicoposterior Segment

The apicoposterior segment is commonly designated S1+S2. It represents the frequent fusion or combined bronchial organization of the apical and posterior segments.

It occupies the superior and posterior portions of the left superior lobe and is supplied by the apicoposterior segmental bronchus, commonly designated B1+2.

Apical and Posterior Components

Although commonly described together as an apicoposterior segment, separate apical and posterior segmental bronchi or territories may occur.

This anatomical variation helps explain why some classifications describe the left lung as having eight segments, while others recognize nine or ten.

Anterior Segment

The anterior segment, designated S3, occupies much of the anterior portion of the superior lobe.

It is supplied by the anterior segmental bronchus, B3, and an accompanying branch of the left pulmonary artery.

Lingula

The lingula forms the anteroinferior portion of the left superior lobe. Functionally and anatomically, it is often compared with the middle lobe of the right lung, although it remains part of the left superior lobe.

The lingula is usually divided into superior and inferior lingular segments.

Superior Lingular Segment

The superior lingular segment is designated S4.

It is supplied by the superior lingular bronchus, B4, and an associated pulmonary arterial branch.

Inferior Lingular Segment

The inferior lingular segment is designated S5.

It lies inferior to S4 and is supplied by the inferior lingular bronchus, B5, together with its corresponding pulmonary arterial branch.

Segments of the Left Superior Lobe

Segment	Designation	Region
Apicoposterior	S1+S2	Superior and posterior portion of superior lobe
Anterior	S3	Anterior superior lobe
Superior lingular	S4	Superior part of lingula
Inferior lingular	S5	Inferior part of lingula

Left Inferior Lobe

The inferior lobe forms much of the posterior and inferior portion of the left lung.

Its bronchopulmonary organization includes a superior segment and several basal segments supplied by branches of the left lower lobe bronchus.

Superior Segment of the Inferior Lobe

The superior segment of the inferior lobe is designated S6.

It occupies the upper posterior portion of the inferior lobe and is supplied by the superior segmental bronchus, B6.

Basal Segments

Inferior to S6, the inferior lobe is divided into basal bronchopulmonary segments.

These segments are organized around branches of the basal bronchial trunk and occupy much of the diaphragmatic and lower peripheral portion of the lung.

Anteromedial Basal Segment

On the left side, the anterior and medial basal territories are frequently combined and described as the anteromedial basal segment, designated S7+S8.

This combined arrangement is one reason the left lung is often described as having fewer independently named bronchopulmonary segments than the right lung.

Anterior Basal Segment

When described separately, the anterior basal segment is designated S8.

It occupies an anterior portion of the basal region of the inferior lobe and is supplied by the anterior basal segmental bronchus, B8.

Medial Basal Segment

A distinct medial basal segment, designated S7, is variable on the left side and may be small, absent as an independent segment, or combined with S8.

Its variability contributes to differences among anatomical descriptions of the left lower lobe.

Lateral Basal Segment

The lateral basal segment, designated S9, occupies the lateral portion of the inferior lobe base.

It is supplied by the lateral basal segmental bronchus, B9.

Posterior Basal Segment

The posterior basal segment, designated S10, occupies the posterior basal region of the inferior lobe.

It is supplied by the posterior basal segmental bronchus, B10.

Segments of the Left Inferior Lobe

Segment	Designation	Region
Superior	S6	Superior posterior part of inferior lobe
Medial basal	S7	Variable medial basal territory
Anterior basal	S8	Anterior basal region
Lateral basal	S9	Lateral basal region
Posterior basal	S10	Posterior basal region

Common Combined Left Lung Classification

Because S1 and S2 are commonly combined and S7 and S8 may also be combined, a frequently used description identifies eight major segmental units in the left lung.

Lobe	Common Segmental Unit	Designation
Superior	Apicoposterior	S1+S2
Superior	Anterior	S3
Superior	Superior lingular	S4
Superior	Inferior lingular	S5
Inferior	Superior	S6
Inferior	Anteromedial basal	S7+S8
Inferior	Lateral basal	S9
Inferior	Posterior basal	S10

Segmental Bronchi

Each bronchopulmonary segment receives a segmental bronchus, also called a tertiary bronchus.

These bronchi arise through progressive branching of the left main bronchus into lobar and then segmental bronchi.

Left Main Bronchus

The left main bronchus enters the root of the left lung and divides into superior and inferior lobar bronchi.

These lobar bronchi subsequently divide into segmental bronchi supplying the individual bronchopulmonary territories.

Pulmonary Arterial Supply

Each bronchopulmonary segment receives an accompanying segmental branch of the pulmonary artery.

The pulmonary arterial branches generally travel with the segmental bronchi within the central portions of the segments.

Pulmonary Venous Drainage

Pulmonary veins differ from the bronchi and pulmonary arteries in their relationship to the segments.

Rather than occupying the center of an individual bronchopulmonary segment, pulmonary venous tributaries commonly travel within intersegmental planes and receive blood from adjacent segments.

Intrasegmental and Intersegmental Structures

Structure	Relationship to Segment
Segmental bronchus	Intrasegmental
Pulmonary arterial branch	Intrasegmental
Pulmonary venous tributaries	Primarily intersegmental

Shape of Bronchopulmonary Segments

Bronchopulmonary segments are often described as pyramidal or wedge-shaped units.

The apex points toward the hilum or root of the lung, where the segmental bronchus enters, while the base is directed toward the pleural surface.

Intersegmental Planes

Connective tissue planes separate neighboring bronchopulmonary segments to varying degrees.

These planes are clinically important because they provide potential surgical dissection boundaries during anatomical segmental lung resection.

Comparison with the Right Lung

The basic principle of bronchopulmonary segmentation is similar in both lungs, but the left lung commonly has more fusion of segmental territories.

Feature	Left Lung	Right Lung
Lobes	2	3
Lingula	Present	Absent
Middle lobe	Absent	Present
S1 and S2	Frequently combined as S1+S2	Usually separate
S7 and S8	May be combined as S7+S8	Usually described separately

Lingula and Right Middle Lobe

The lingula is frequently compared anatomically with the right middle lobe because both occupy an anterior region of their respective lungs and contain two commonly described bronchopulmonary segments.

However, the lingula belongs to the left superior lobe and is not a separate lobe.

Anatomical Variation

Bronchial branching patterns show substantial individual variation. Segmental bronchi may arise from common trunks, divide differently, or supply territories that do not precisely match simplified textbook diagrams.

For this reason, descriptions of the left lung may recognize eight, nine, or ten bronchopulmonary segments depending on whether particular territories are considered fused or separate.

Radiological Anatomy

Knowledge of bronchopulmonary segments is important when interpreting chest radiographs and cross-sectional imaging such as computed tomography.

Localization of pulmonary lesions to a particular segment can help determine their relationship to segmental bronchi, pulmonary arteries, veins, fissures, and adjacent lobes.

Bronchoscopy

During bronchoscopy, the branching pattern of lobar and segmental bronchi provides an anatomical roadmap for examining specific regions of the lung.

Identification of the correct segmental bronchial opening allows targeted inspection, sampling, lavage, or other procedures.

Segmentectomy

A segmentectomy is surgical removal of one or more bronchopulmonary segments while preserving neighboring pulmonary tissue.

The segmental organization of the lung makes this possible because each segment has a relatively independent bronchial and pulmonary arterial supply.

Surgical Importance of Pulmonary Veins

Because pulmonary venous tributaries often lie between segments, they can help identify intersegmental boundaries during surgery.

Preserving appropriate venous drainage of the remaining lung tissue is an important consideration during anatomical segmentectomy.

Localization of Disease

Some pulmonary diseases may predominantly involve particular bronchopulmonary segments because inhaled or aspirated material, infection, tumors, or obstructive processes can follow segmental bronchial pathways.

Describing disease by segment provides more precise anatomical localization than identifying the affected lobe alone.

Postural and Positional Relationships

The distribution of aspirated material within the bronchial tree is influenced by gravity, body position, and bronchial anatomy.

Knowledge of individual segmental positions is therefore useful when evaluating aspiration-related disease and dependent pulmonary abnormalities.

Segmental Organization

1. The left main bronchus enters the left lung.
2. It divides into superior and inferior lobar bronchi.
3. Lobar bronchi give rise to segmental bronchi.
4. Each segmental bronchus supplies a bronchopulmonary segment.
5. A corresponding pulmonary arterial branch accompanies the bronchus.
6. Pulmonary veins generally course between adjacent segments.
7. The superior lobe includes apicoposterior, anterior, and lingular territories.
8. The inferior lobe includes superior and basal territories.
9. Some segmental territories are commonly combined on the left side.

Key Anatomical Relationships

• The left lung contains superior and inferior lobes.
• The oblique fissure separates the two lobes.
• Bronchopulmonary segments are supplied by segmental bronchi.
• Segmental pulmonary arterial branches accompany the bronchi.
• Pulmonary veins commonly occupy intersegmental planes.
• The apical and posterior segments are frequently combined as S1+S2.
• S3 is the anterior segment of the superior lobe.
• S4 and S5 form the superior and inferior lingular segments.
• S6 is the superior segment of the inferior lobe.
• S7 may be absent as a distinct segment or combined with S8.
• S8 represents the anterior basal territory when described separately.
• S9 is the lateral basal segment.
• S10 is the posterior basal segment.
• The left lung may be described as having approximately eight to ten segments depending on classification.
• Segmental anatomy provides the basis for anatomical segmentectomy.

Clinical Importance

The bronchopulmonary segments of the left lung provide a detailed anatomical framework for understanding the distribution of bronchi, pulmonary arteries, pulmonary veins, and pulmonary parenchyma. Their organization permits precise localization of abnormalities within the lung and provides a practical map for radiological interpretation, bronchoscopy, and surgery.

The left lung differs from the right because several segmental territories are commonly combined. The apical and posterior territories frequently form an apicoposterior segment, while the medial and anterior basal territories may form an anteromedial basal segment. Consequently, the number of separately named left lung segments varies among classification systems.

Understanding left lung segments provides the anatomical foundation for studying segmental bronchi, pulmonary arterial branches, intersegmental veins, the lingula, bronchopulmonary segmentectomy, bronchoscopic localization, and segmental patterns of pulmonary disease.