Left Lung (2 Lobes)

The left lung is one of the paired respiratory organs occupying the thoracic cavity. It is divided into two lobes, the superior and inferior lobes, which are separated by an oblique fissure. The left lung is generally smaller and narrower than the right lung because the heart projects predominantly into the left side of the thoracic cavity.

Characteristic anatomical features of the left lung include a prominent cardiac impression, a cardiac notch along its anterior border, and the lingula, a tongue-shaped projection of the superior lobe. These features distinguish it from the three-lobed right lung.

Lobes of the Left Lung

The left lung contains two lobes:

• Superior lobe
• Inferior lobe

The lobes are separated by the oblique fissure. Unlike the right lung, the left lung normally has no horizontal fissure and no separate middle lobe.

Superior Lobe

The superior lobe occupies the upper portion of the left lung and extends from the apex downward to the oblique fissure.

Anteriorly, it extends farther inferiorly than it does posteriorly because of the oblique course of the fissure. The superior lobe also includes the lingula, which projects inferiorly adjacent to the cardiac notch.

Inferior Lobe

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

Although it contributes relatively little to the anterior surface of the lung, it forms a large part of the posterior pulmonary surface and rests inferiorly upon the diaphragm.

Oblique Fissure

The oblique fissure separates the superior and inferior lobes of the left lung.

It passes obliquely across the pulmonary surface, extending from a relatively high posterior position downward and anteriorly toward the inferior border.

Function of the Oblique Fissure

The fissure allows the superior and inferior lobes to move relative to one another during respiration.

Visceral pleura extends into the fissure and covers the opposing surfaces of the two lobes.

Absence of a Horizontal Fissure

The left lung normally lacks the horizontal fissure found in the right lung.

As a result, the left lung contains only superior and inferior lobes rather than the superior, middle, and inferior lobes present on the right.

Lingula

The lingula is a tongue-shaped projection from the anteroinferior portion of the left superior lobe.

It lies inferior to the cardiac notch and overlies part of the heart. The lingula is commonly regarded as the anatomical counterpart of the right middle lobe, although it is not a separate lobe.

Lingular Segments

The lingula contains two commonly recognized bronchopulmonary segments:

• Superior lingular segment, S4
• Inferior lingular segment, S5

These are supplied by corresponding segmental bronchi arising from the left superior lobar bronchus.

Cardiac Notch

The cardiac notch is a prominent indentation in the anterior border of the left lung.

It accommodates the leftward projection of the heart and contributes to the characteristic shape of the left superior lobe.

Cardiac Impression

The mediastinal surface of the left lung bears a deep cardiac impression.

This depression is produced by the heart and pericardium and is deeper and more extensive than the cardiac impression on the right lung.

Size of the Left Lung

The left lung is generally smaller in volume than the right lung because of the position of the heart.

It is typically narrower than the right lung, while the right lung is generally somewhat shorter because the right dome of the diaphragm is elevated by the liver.

Apex

The apex is the rounded superior extremity of the left lung.

It extends into the root of the neck above the level of the first rib and is covered by cervical pleura.

Base

The base of the left lung is formed by its diaphragmatic surface.

It is concave and conforms to the superior surface of the left dome of the diaphragm.

Surfaces of the Left Lung

The left lung is commonly described as having three major surfaces:

• Costal surface
• Mediastinal surface
• Diaphragmatic surface

These surfaces reflect the major structures surrounding the lung within the thoracic cavity.

Costal Surface

The costal surface is smooth, broad, and convex.

It faces the ribs and intercostal spaces and conforms to the curvature of the internal thoracic wall.

Mediastinal Surface

The mediastinal surface faces medially toward the mediastinum and contains the hilum of the lung.

It is more irregular than the costal surface because it is molded by the heart, aorta, esophagus, and other mediastinal structures.

Diaphragmatic Surface

The diaphragmatic surface is concave and forms the base of the lung.

It rests upon the diaphragm, which separates the lung from abdominal structures below.

Borders of the Left Lung

The major borders of the left lung are:

• Anterior border
• Posterior border
• Inferior border

These borders mark transitions between the major pulmonary surfaces.

Anterior Border

The anterior border is relatively thin and sharp and separates the costal and mediastinal surfaces anteriorly.

On the left lung it is interrupted by the cardiac notch, below which the lingula projects.

Posterior Border

The posterior border is broad and rounded and lies alongside the thoracic vertebral column.

It forms a gradual transition between the posterior parts of the costal and mediastinal surfaces.

Inferior Border

The inferior border separates the diaphragmatic surface from the costal and mediastinal surfaces.

Its position changes during respiration as the lung expands and recoils with movement of the diaphragm.

Hilum of the Left Lung

The hilum is located on the mediastinal surface and forms the gateway through which structures of the lung root enter and leave the lung.

These structures include the left main bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics, lymph nodes, and autonomic nerves.

Root of the Left Lung

The root of the left lung is the bundle of structures connecting the lung to the mediastinum.

Near the lung, the root is surrounded by a pleural sleeve formed by reflection of mediastinal pleura onto the pulmonary surface.

Arrangement at the Left Root

A characteristic feature of the left root is that the left pulmonary artery lies superior to the left main bronchus.

A simplified superior-to-inferior arrangement is:

1. Left pulmonary artery
2. Left main bronchus
3. Inferior pulmonary vein

The superior pulmonary vein occupies a relatively anterior position and is not adequately represented by a purely vertical sequence.

Left Pulmonary Artery

The left pulmonary artery carries deoxygenated blood from the pulmonary trunk toward the left lung.

At the root, it lies superior to the left main bronchus before dividing into branches that accompany the bronchial tree.

Left Pulmonary Veins

The left superior and inferior pulmonary veins return oxygenated blood from the left lung to the left atrium.

The veins occupy relatively anterior and inferior positions within the root, with the inferior pulmonary vein forming the lowest major component.

Left Main Bronchus

The left main bronchus is generally longer, narrower, and more oblique than the right main bronchus.

It enters the lung at the hilum and divides into superior and inferior lobar bronchi supplying the two lobes.

Superior Lobar Bronchus

The left superior lobar bronchus supplies the superior lobe, including the lingula.

It ultimately gives rise to segmental bronchi supplying the apicoposterior, anterior, superior lingular, and inferior lingular territories.

Inferior Lobar Bronchus

The left inferior lobar bronchus supplies the inferior lobe.

It gives rise to a superior segmental bronchus and basal segmental branches supplying the lower portion of the lung.

Bronchopulmonary Segments

The lobes of the left lung are subdivided into bronchopulmonary segments. Each segment is supplied by a segmental bronchus and a corresponding branch of the pulmonary artery.

The left lung is commonly described as having approximately eight to ten segments because some territories frequently share bronchial trunks or are described as combined segments.

Superior Lobe Segments

A commonly used arrangement of the superior lobe includes:

• Apicoposterior segment, S1+S2
• Anterior segment, S3
• Superior lingular segment, S4
• Inferior lingular segment, S5

The apical and posterior territories may occasionally be described separately.

Inferior Lobe Segments

The inferior lobe includes:

• Superior segment, S6
• Medial basal segment, S7, when present separately
• Anterior basal segment, S8
• Lateral basal segment, S9
• Posterior basal segment, S10

The medial and anterior basal territories may be combined and described as an anteromedial basal segment, S7+S8.

Segmental Variation

The number of separately named bronchopulmonary segments in the left lung varies among anatomical classifications.

This variation results particularly from frequent combination of the apical and posterior territories and variable separation of the medial and anterior basal territories.

Fissure and Segmental Boundaries

The oblique fissure separates pulmonary lobes, but bronchopulmonary segments are subdivisions within the lobes and are not separated by major pleural fissures.

Segmental boundaries are defined primarily by bronchovascular organization and intersegmental connective tissue planes.

Pulmonary Venous Drainage

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

Venous tributaries commonly pass within intersegmental planes and receive blood from adjacent segments before converging toward the left pulmonary veins.

Visceral Pleura

The entire external surface of the left lung is covered by visceral pleura, except at the hilum where structures enter and leave.

The visceral pleura also extends deeply into the oblique fissure and covers the opposing surfaces of the superior and inferior lobes.

Mediastinal Impressions

The left lung is molded by several structures within the mediastinum.

Important impressions and grooves include those associated with the heart, arch of the aorta, descending thoracic aorta, and left subclavian artery.

Aortic Arch Groove

The arch of the aorta passes superior to the root of the left lung and produces a curved groove on the mediastinal surface.

This groove is an important distinguishing feature of the left lung.

Descending Aortic Groove

The descending thoracic aorta lies posterior to the left lung root.

It produces a prominent vertical groove on the posterior portion of the mediastinal surface.

Left Subclavian Artery Groove

The left subclavian artery ascends toward the root of the neck and may produce a groove on the superior medial surface near the apex of the left lung.

The prominence of this impression varies among individuals.

Relationship to the Heart

The heart occupies a substantial portion of the middle mediastinum and projects more prominently toward the left side.

This relationship accounts for the smaller volume of the left lung, the deep cardiac impression, and the cardiac notch along the anterior border.

Relationship to the Diaphragm

The base of the left lung rests upon the left dome of the diaphragm.

The diaphragm separates the lung from upper abdominal viscera, particularly the stomach and spleen on the left side, although these organs do not directly contact the lung because the diaphragm and pleural structures intervene.

Relationship to the Thoracic Wall

The convex costal surface conforms to the ribs and intercostal spaces.

During inspiration, expansion of the thoracic cage and descent of the diaphragm increase thoracic volume, allowing the left lung to expand.

Comparison with the Right Lung

Feature	Left Lung	Right Lung
Number of lobes	2	3
Oblique fissure	Present	Present
Horizontal fissure	Absent	Present
Cardiac notch	Present	Absent
Lingula	Present	Absent
Cardiac impression	Deeper	Shallower
Common segment count	Approximately 8 to 10	10

Lingula and Right Middle Lobe

The lingula is often compared with the right middle lobe because both occupy anterior regions of their respective lungs and each contains two commonly recognized bronchopulmonary segments.

However, the lingula remains part of the left superior lobe and is not separated from it by a complete fissure.

Surface Projection

The left lung extends from its apex in the root of the neck to its base on the diaphragm.

Its inferior border moves vertically during respiration, descending during inspiration and ascending during expiration.

Left Lung on Imaging

The two-lobed organization of the left lung can be evaluated using chest radiography and particularly computed tomography.

Cross-sectional imaging demonstrates the oblique fissure, lobar bronchi, pulmonary vessels, bronchopulmonary segments, hilum, and relationships with the heart and mediastinum.

Oblique Fissure on Imaging

The oblique fissure provides an important landmark for determining whether pulmonary abnormalities lie within the superior or inferior lobe.

Its visibility varies according to imaging plane, slice thickness, lung expansion, and individual anatomy.

Lobar Localization of Disease

Pneumonia, atelectasis, tumors, pulmonary edema, and other disorders may affect individual lobes or bronchopulmonary segments.

Knowledge of the oblique fissure and lobar anatomy allows abnormalities to be localized more accurately within the left lung.

Lingula in Clinical Anatomy

Because the lingula lies adjacent to the heart, abnormalities involving it may alter the left cardiac border on frontal chest radiographs.

Localization of disease to the lingula can therefore be aided by understanding its anatomical relationship with the cardiac silhouette.

Bronchoscopy

During bronchoscopy, the left main bronchus can be followed into superior and inferior lobar bronchi.

Further branching provides access to individual segmental bronchi and allows targeted inspection, lavage, sampling, or treatment of specific regions of the left lung.

Lobectomy

A lobectomy involves surgical removal of an entire pulmonary lobe.

On the left side, this may involve removal of either the superior or inferior lobe while preserving the remaining lobe and its bronchovascular supply.

Segmentectomy

A segmentectomy removes one or more bronchopulmonary segments rather than an entire lobe.

The relatively independent segmental bronchial and arterial supply permits selected segments to be removed while preserving surrounding pulmonary tissue.

Pneumonectomy

A left pneumonectomy involves removal of the entire left lung.

The procedure requires control and division of major structures at the left lung root, including the main bronchus, pulmonary artery, and pulmonary veins.

Key Anatomical Relationships

• The left lung contains two lobes.
• The superior and inferior lobes are separated by the oblique fissure.
• The left lung normally lacks a horizontal fissure.
• The superior lobe includes the lingula.
• The lingula lies inferior to the cardiac notch.
• The cardiac notch interrupts the anterior border.
• The cardiac impression is deeper on the left lung than on the right.
• The mediastinal surface contains the pulmonary hilum.
• The left pulmonary artery lies superior to the left main bronchus at the root.
• The left main bronchus divides into superior and inferior lobar bronchi.
• The left lung commonly has approximately eight to ten bronchopulmonary segments.
• The arch of the aorta passes above the left lung root.
• The descending thoracic aorta lies posterior to the root.
• The costal surface faces the thoracic wall.
• The diaphragmatic surface forms the base of the lung.
• Visceral pleura covers the lung and extends into the oblique fissure.

Clinical Importance

The two-lobed organization of the left lung reflects its adaptation to the position of the heart within the thorax. The deep cardiac impression, cardiac notch, and lingula are characteristic features produced by this relationship and provide important landmarks for identifying the left lung.

The oblique fissure separates the superior and inferior lobes and permits relative movement between them during respiration. Within each lobe, bronchopulmonary segments provide more detailed anatomical subdivisions based on the distribution of segmental bronchi and pulmonary arterial branches.

Understanding the left lung provides the anatomical foundation for studying pulmonary lobes, oblique fissure, lingula, cardiac notch, bronchopulmonary segments, pulmonary hilum, lung root, pleural relationships, thoracic imaging, bronchoscopy, and pulmonary surgery.