Hilum of Lung

The hilum of the lung is a depressed, approximately wedge-shaped region on the mediastinal surface of each lung where structures forming the root of the lung enter and leave the pulmonary tissue. It serves as the gateway between the lung and the mediastinum.

Structures passing through the hilum include the main or lobar bronchi, pulmonary arteries, pulmonary veins, bronchial arteries and veins, lymphatic vessels, lymph nodes, and autonomic nerves. Their arrangement differs between the right and left lungs because of differences in bronchial branching and relationships with surrounding mediastinal structures.

Location

The hilum is located on the mediastinal surface of each lung. It lies approximately in the central region of this surface and faces the mediastinum.

The mediastinal pleura reflects from the mediastinum onto the lung around the structures passing through the hilum.

Hilum and Root of the Lung

The terms hilum and root of the lung are closely related but describe different anatomical concepts.

The hilum is the area on the lung surface through which structures pass. The root is the collection of structures that actually enter or leave the lung through this region and connect the lung to the mediastinum.

Hilum versus Root

Feature	Hilum	Root
Definition	Depressed area on the mediastinal surface of the lung	Bundle of structures connecting the lung with the mediastinum
Function	Provides the passageway into and out of the lung	Carries airways, vessels, nerves, and lymphatics
Pleural relationship	Surrounded by reflection of mediastinal pleura	Enclosed by a pleural sleeve near the lung

Contents of the Hilum

The principal structures passing through the pulmonary hilum include:

• Main or lobar bronchi
• Pulmonary artery and its branches
• Superior and inferior pulmonary veins
• Bronchial arteries
• Bronchial veins
• Lymphatic vessels
• Bronchopulmonary lymph nodes
• Autonomic nerve fibers forming the pulmonary plexuses
• Connective tissue

Bronchi at the Hilum

The bronchi carry air between the trachea and the lungs. The right and left main bronchi approach their respective lungs and undergo further branching in relation to the hilum.

The bronchial arrangement differs between the two sides, particularly because the right superior lobar bronchus arises relatively early and has a distinctive relationship to the right pulmonary artery.

Pulmonary Arteries

The right and left pulmonary arteries transport deoxygenated blood from the pulmonary trunk toward the lungs.

At or near the hilum, the pulmonary arteries divide into branches that accompany the bronchial tree and ultimately supply the pulmonary capillary networks surrounding the alveoli.

Pulmonary Veins

Each lung normally drains through a superior pulmonary vein and an inferior pulmonary vein.

The pulmonary veins carry oxygenated blood from the lungs to the left atrium. At the hilum they generally occupy relatively anterior and inferior positions compared with the major bronchial structures.

Bronchial Vessels

The bronchial arteries provide systemic arterial blood to the bronchi, connective tissue, visceral pleura, and supporting structures of the lungs.

Bronchial veins drain part of this systemic circulation. These smaller vessels accompany the larger bronchovascular structures through the root and hilum.

Lymphatics

Lymphatic vessels from the lung converge toward lymph nodes situated along the bronchi and near the hilum.

Lymph ultimately passes through pulmonary and bronchopulmonary, or hilar, lymph nodes before continuing toward tracheobronchial lymph nodes and more central lymphatic pathways.

Bronchopulmonary Lymph Nodes

Bronchopulmonary lymph nodes are located around the lobar bronchi and near the hilum of each lung.

They receive lymph from pulmonary lymphatic networks and are important in the spread and radiological assessment of pulmonary infection, inflammatory disease, and malignancy.

Autonomic Nerves

Autonomic nerves reach the lungs through the pulmonary plexuses, which are located around the roots of the lungs.

These plexuses contain sympathetic fibers, parasympathetic fibers derived from the vagus nerves, and visceral afferent fibers.

Pleural Reflection

The mediastinal pleura reflects around the structures forming the root of the lung and becomes continuous with the visceral pleura covering the pulmonary surface.

This reflection forms a pleural sleeve around the root and defines the boundary between mediastinal and visceral pleura in the hilar region.

Pulmonary Ligament

Inferior to the root, the anterior and posterior layers of the pleural reflection extend downward as the pulmonary ligament.

The pulmonary ligament is a double layer of pleura that allows some movement of the root structures and accommodates changes in the position of the pulmonary veins and lungs during respiration.

Right Hilum

The arrangement of structures at the right hilum reflects the branching pattern of the right main bronchus and its relationship with the right pulmonary artery.

A particularly important feature is the presence of the right superior lobar bronchus above the pulmonary artery.

Eparterial Bronchus

The right superior lobar bronchus is termed the eparterial bronchus because it lies superior to the right pulmonary artery at the hilum.

This relationship is characteristic of the right lung and is useful when comparing right and left hilar anatomy.

Hyparterial Bronchi

Bronchial branches lying below the pulmonary artery are described as hyparterial bronchi.

On the right side, the bronchus intermedius continues inferior to the origin of the superior lobar bronchus and ultimately supplies the middle and inferior lobes.

Right Hilum from Superior to Inferior

A useful simplified superior-to-inferior arrangement at the right hilum is:

1. Eparterial bronchus
2. Pulmonary artery
3. Hyparterial bronchial structures
4. Inferior pulmonary vein

The exact appearance varies with the level and perspective from which the hilum is examined.

Left Hilum

At the left hilum, the pulmonary artery occupies a position superior to the main bronchial structure.

This contrasts with the right side, where the eparterial superior lobar bronchus lies above the pulmonary artery.

Left Hilum from Superior to Inferior

A simplified superior-to-inferior arrangement at the left hilum is:

1. Pulmonary artery
2. Main bronchus
3. Inferior pulmonary vein

The superior pulmonary vein lies more anteriorly and is not adequately represented by a purely superior-to-inferior sequence.

Relationship of Pulmonary Artery and Bronchus

A commonly used anatomical relationship is remembered by the mnemonic RALS: Right Anterior, Left Superior.

This refers to the relationship of the pulmonary artery to the main bronchus: the right pulmonary artery is generally anterior to the bronchus, while the left pulmonary artery is generally superior to the bronchus.

Anterior-to-Posterior Arrangement

Although hilar anatomy is three-dimensional and cannot be completely represented by a single sequence, the pulmonary veins generally occupy relatively anterior positions, while bronchial structures tend to lie more posteriorly.

The pulmonary arteries occupy intermediate relationships that differ between the right and left sides.

Right and Left Hilar Differences

Feature	Right Hilum	Left Hilum
Highest major structure	Eparterial superior lobar bronchus	Pulmonary artery
Pulmonary artery relative to bronchus	Generally anterior	Generally superior
Eparterial bronchus	Present	Absent
Inferior pulmonary vein	Inferior part of hilum	Inferior part of hilum

Relations of the Right Root

The right lung root lies in close relationship with several mediastinal structures.

• The phrenic nerve passes anterior to the root.
• The vagus nerve passes posterior to the root.
• The azygos vein arches superior to the root before entering the superior vena cava.

Relations of the Left Root

The left lung root also has important mediastinal relationships.

• The phrenic nerve passes anterior to the root.
• The vagus nerve passes posterior to the root.
• The arch of the aorta passes superior to the root.
• The descending thoracic aorta lies posterior to the left lung root.

Phrenic Nerves

The phrenic nerves descend through the thorax anterior to the roots of the lungs.

This relationship is an important landmark for distinguishing the course of the phrenic nerves from that of the vagus nerves.

Vagus Nerves

The vagus nerves pass posterior to the roots of the lungs.

They contribute parasympathetic and visceral afferent fibers to the pulmonary plexuses before continuing into the thorax.

Relationship to the Mediastinum

The hilum provides the interface between pulmonary structures and the mediastinum. Airways, vessels, nerves, and lymphatics converge in this relatively compact region.

Because of this concentration of structures, hilar abnormalities can affect several components of the respiratory and cardiovascular systems simultaneously.

Hilar Anatomy on Imaging

The hilar regions are important landmarks on chest radiographs and computed tomography examinations.

The visible hilar shadows are produced largely by pulmonary vessels, particularly the pulmonary arteries and their branches. Bronchi, lymph nodes, and other structures also contribute to hilar anatomy but may be less conspicuous under normal conditions.

Left and Right Hilar Levels

On frontal chest imaging, the left hilum commonly appears slightly higher than the right hilum because of differences in pulmonary arterial anatomy.

Assessment of hilar position, size, contour, and density can provide information about pulmonary vascular and mediastinal abnormalities.

Hilar Enlargement

Apparent enlargement of the pulmonary hilum can result from enlargement of pulmonary vessels, lymph nodes, or masses arising in or adjacent to the hilar region.

Cross-sectional imaging is often used to determine which anatomical structure is responsible for an abnormal hilar contour.

Hilar Lymphadenopathy

Enlargement of bronchopulmonary and other hilar lymph nodes is termed hilar lymphadenopathy.

Hilar lymph nodes may enlarge in association with infections, inflammatory disorders, granulomatous disease, and malignancy. The distribution and associated findings help determine the clinical significance of lymph node enlargement.

Lung Cancer and the Hilum

Central pulmonary tumors may involve bronchi, pulmonary vessels, lymph nodes, or other structures near the hilum.

Knowledge of hilar anatomy is therefore important when assessing tumor location, bronchovascular involvement, lymphatic spread, and surgical resectability.

Bronchoscopy and Hilar Anatomy

Bronchoscopy provides an internal view of the bronchial tree rather than the external hilum itself, but bronchial branching at the root and hilar region determines the pathways used to access the pulmonary lobes and segments.

Understanding these relationships helps correlate bronchoscopic findings with imaging and surgical anatomy.

Surgical Importance

Operations involving lobectomy, pneumonectomy, transplantation, and other pulmonary procedures require precise identification of structures within the lung root and hilum.

The pulmonary arteries, pulmonary veins, bronchi, bronchial vessels, lymphatics, and nerves must be distinguished because their arrangement differs between the two sides and may show individual variation.

Functional Organization of Hilar Structures

Structure	Primary Function
Bronchi	Conduct air into and out of the lung
Pulmonary arteries	Carry deoxygenated blood to the lung
Pulmonary veins	Return oxygenated blood to the left atrium
Bronchial vessels	Provide systemic vascular supply and drainage to supporting pulmonary tissues
Lymphatics	Drain lymph from pulmonary tissues
Autonomic nerves	Provide autonomic and visceral sensory innervation

Key Anatomical Relationships

• The hilum is located on the mediastinal surface of each lung.
• The root of the lung passes through the hilum.
• The hilum transmits bronchi, pulmonary vessels, bronchial vessels, lymphatics, and nerves.
• The pulmonary veins generally occupy relatively anterior and inferior positions.
• Bronchial structures tend to lie relatively posteriorly.
• The right superior lobar bronchus lies above the right pulmonary artery and is termed eparterial.
• The right pulmonary artery is generally anterior to the bronchus.
• The left pulmonary artery is generally superior to the bronchus.
• The inferior pulmonary vein occupies an inferior position in each root.
• The phrenic nerve passes anterior to each lung root.
• The vagus nerve passes posterior to each lung root.
• The azygos arch passes superior to the right lung root.
• The aortic arch passes superior to the left lung root.
• Mediastinal pleura reflects around the root onto the lung.
• The pulmonary ligament extends inferiorly from the pleural sleeve surrounding the root.

Clinical Importance

The hilum is one of the most anatomically concentrated regions of the lung because the major airways, pulmonary vessels, systemic bronchial vessels, lymphatics, lymph nodes, and autonomic nerves converge within a relatively small area. Accurate understanding of these relationships is essential for interpreting thoracic imaging and performing pulmonary surgery.

The right and left hilar arrangements are not identical. On the right, the superior lobar bronchus lies above the pulmonary artery, whereas on the left the pulmonary artery lies above the bronchus. The relationship of the pulmonary arteries to the bronchi, together with the positions of the pulmonary veins, provides important landmarks for identifying hilar structures.

Understanding the hilum of the lung provides the anatomical foundation for studying the lung root, pulmonary arteries, pulmonary veins, main and lobar bronchi, bronchopulmonary lymph nodes, pulmonary plexuses, pulmonary ligament, hilar imaging, and surgical approaches to the lung.