Root of Lung

The root of the lung is the collection of structures that connect each lung to the mediastinum. These structures enter or leave the lung through the hilum on its mediastinal surface and include the bronchi, pulmonary arteries, pulmonary veins, bronchial vessels, lymphatic vessels, lymph nodes, autonomic nerves, and supporting connective tissue.

The lung root is surrounded by a sleeve of pleura formed by reflections of mediastinal pleura onto the surface of the lung. Inferior to the root, this pleural reflection continues downward as the pulmonary ligament.

Location

The root of each lung lies between the lung and the mediastinum. It approaches the medial surface of the lung and passes through the pulmonary hilum.

The structures within the root extend from mediastinal structures toward the lung and then branch extensively within the pulmonary tissue.

Root and Hilum

The root and hilum are closely related but are not synonymous.

The hilum is the depressed area on the mediastinal surface of the lung through which structures pass. The root is the actual bundle of structures passing between the lung and mediastinum through this area.

Root versus Hilum

Feature	Root of Lung	Hilum of Lung
Definition	Bundle of structures connecting lung and mediastinum	Area on the mediastinal surface through which the root passes
Components	Bronchi, vessels, nerves, lymphatics, and connective tissue	Surface gateway for these structures
Pleural relationship	Surrounded by a pleural sleeve near the lung	Bounded by the pleural reflection

Contents of the Root

The principal structures forming the root of each lung include:

• Main and lobar bronchi
• Pulmonary artery and its branches
• Superior pulmonary vein
• Inferior pulmonary vein
• Bronchial arteries
• Bronchial veins
• Lymphatic vessels
• Bronchopulmonary lymph nodes
• Sympathetic nerve fibers
• Parasympathetic fibers from the vagus nerve
• Visceral afferent fibers
• Connective tissue

Bronchi

The main bronchi form major components of the lung roots. The right and left main bronchi arise from the bifurcation of the trachea and travel toward their respective lungs.

Near and within the roots, the bronchi branch into lobar bronchi and subsequently into progressively smaller divisions supplying the pulmonary lobes and bronchopulmonary segments.

Right Main Bronchus

The right main bronchus is generally shorter, wider, and more vertical than the left main bronchus.

Its superior lobar bronchus arises relatively early and lies superior to the right pulmonary artery, giving it an important anatomical relationship within the right lung root.

Eparterial Bronchus

The right superior lobar bronchus is known as the eparterial bronchus because it lies above the right pulmonary artery.

This is a characteristic feature of the right root and helps distinguish the arrangement of bronchial and vascular structures on the two sides.

Bronchus Intermedius

After giving rise to the superior lobar bronchus, the right main bronchus continues as the bronchus intermedius.

The bronchus intermedius passes inferiorly and gives rise to bronchi supplying the middle and inferior lobes.

Left Main Bronchus

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

At the left root, the pulmonary artery lies superior to the main bronchus, producing a different bronchovascular arrangement from that seen on the right.

Pulmonary Arteries

The pulmonary arteries carry deoxygenated blood from the right ventricle through the pulmonary trunk to the lungs.

The right and left pulmonary arteries enter their respective lung roots and divide into lobar and segmental branches that accompany the bronchial tree.

Right Pulmonary Artery

The right pulmonary artery passes horizontally toward the right lung and has important relationships with the superior vena cava, ascending aorta, and right main bronchial structures.

At the root, the right pulmonary artery lies generally anterior to the bronchus, except that the eparterial superior lobar bronchus lies above it.

Left Pulmonary Artery

The left pulmonary artery passes toward the left lung and arches over the left main bronchus as it approaches the pulmonary root.

At the left root, the pulmonary artery occupies a position superior to the bronchus.

RALS Relationship

The relationship of the pulmonary arteries to the bronchi can be remembered using the mnemonic RALS: Right Anterior, Left Superior.

The right pulmonary artery is generally anterior to the right bronchus, whereas the left pulmonary artery is generally superior to the left bronchus.

Pulmonary Veins

Each lung normally has two major pulmonary veins, a superior pulmonary vein and an inferior pulmonary vein.

These vessels carry oxygenated blood from the lungs to the left atrium and form important anterior and inferior components of the lung roots.

Superior Pulmonary Veins

The superior pulmonary veins drain pulmonary territories of the upper portions of the lungs and, on the right, also receive drainage associated with the middle lobe.

Within the root, the superior pulmonary vein generally occupies a relatively anterior position.

Inferior Pulmonary Veins

The inferior pulmonary veins drain the inferior lobes and occupy the lowest major vascular positions within the lung roots.

Their inferior location is an important landmark in hilar and surgical anatomy.

Bronchial Arteries

The bronchial arteries provide systemic arterial blood to the walls of the bronchi, supporting connective tissues, visceral pleura, and other non-respiratory portions of the lung.

These vessels enter the lungs through the roots and accompany branches of the bronchial tree.

Bronchial Veins

Bronchial veins drain part of the systemic blood supplied by the bronchial arteries.

They accompany bronchial structures toward the lung roots, although a substantial portion of bronchial circulation also drains through connections with pulmonary veins.

Lymphatic Vessels

Lymphatic vessels from the lung converge toward the pulmonary root.

Superficial and deep pulmonary lymphatic networks ultimately drain toward pulmonary and bronchopulmonary lymph nodes before lymph continues toward tracheobronchial nodes.

Bronchopulmonary Lymph Nodes

Bronchopulmonary lymph nodes, also called hilar lymph nodes, are located around the major bronchi and structures of the lung root.

They receive lymph from pulmonary lymphatic vessels and form important stations in lymphatic drainage from the lungs.

Pulmonary Plexuses

Autonomic nerves associated with each root form anterior and posterior pulmonary plexuses.

These plexuses contain sympathetic fibers, parasympathetic fibers from the vagus nerves, and visceral afferent fibers that distribute along the bronchial tree and pulmonary vessels.

Sympathetic Fibers

Sympathetic fibers reach the pulmonary plexuses primarily through branches from thoracic sympathetic ganglia.

Adrenergic influences contribute to regulation of bronchial smooth muscle, vascular tone, and glandular activity.

Parasympathetic Fibers

Parasympathetic fibers reaching the pulmonary roots arise from the vagus nerves.

These fibers contribute to bronchoconstriction and increased glandular secretion and participate in autonomic respiratory reflexes.

Visceral Afferent Fibers

Visceral afferent fibers accompany autonomic pathways from the lungs and respiratory passages.

They transmit sensory information involved in pulmonary reflexes and visceral sensation.

Pleural Sleeve

The structures of the lung root are surrounded near the lung by a tubular reflection of pleura often described as a pleural sleeve.

The mediastinal pleura reflects laterally over the root and becomes continuous with the visceral pleura covering the lung.

Pulmonary Ligament

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

The pulmonary ligament is a double layer of pleura that provides mobility for structures at the root and accommodates movement of the lungs and pulmonary veins during respiration.

Arrangement of the Right Lung Root

The right root has a distinctive arrangement because the superior lobar bronchus lies above the pulmonary artery.

A simplified superior-to-inferior arrangement is:

1. Eparterial superior lobar bronchus
2. Right pulmonary artery
3. Hyparterial bronchial structures
4. Inferior pulmonary vein

This sequence is a simplified representation because the structures of the root occupy three-dimensional anterior, posterior, superior, and inferior relationships.

Arrangement of the Left Lung Root

At the left root, the pulmonary artery lies above the bronchus.

A simplified superior-to-inferior arrangement is:

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

The superior pulmonary vein lies relatively anteriorly and therefore is not adequately represented by a purely vertical sequence.

Anterior-to-Posterior Relationships

In both lung roots, pulmonary veins tend to occupy relatively anterior positions, while bronchial structures lie more posteriorly.

The pulmonary arteries occupy intermediate relationships that differ between the two sides.

Right and Left Root Comparison

Feature	Right Root	Left Root
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	Inferior
Pulmonary ligament	Extends inferiorly	Extends inferiorly

Phrenic Nerve Relationship

The phrenic nerve passes anterior to the root of the lung on both sides.

This relationship is particularly important in thoracic surgery and provides a useful landmark for distinguishing the phrenic nerve from the vagus nerve.

Vagus Nerve Relationship

The vagus nerve passes posterior to the lung root on both sides.

Branches from the vagus contribute to the pulmonary plexuses before the main vagal trunks continue inferiorly.

Phrenic and Vagus Nerve Relationship

Nerve	Relationship to Lung Root
Phrenic nerve	Passes anterior to the root
Vagus nerve	Passes posterior to the root

Relations of the Right Lung Root

The right lung root is related to several important mediastinal structures.

• The phrenic nerve passes anteriorly.
• The vagus nerve passes posteriorly.
• The arch of the azygos vein passes superior to the root.
• The superior vena cava lies anterior to the upper portion of the root.
• The right atrium lies anterior and inferior to the root.

Arch of the Azygos Vein

The azygos vein arches forward over the superior aspect of the right lung root before entering the superior vena cava.

This relationship is a characteristic landmark of the right mediastinum.

Relations of the Left Lung Root

The left lung root is closely related to major structures of the left mediastinum.

• The phrenic nerve passes anteriorly.
• The vagus nerve passes posteriorly.
• The arch of the aorta passes superiorly.
• The descending thoracic aorta lies posteriorly.
• The heart and pericardium lie anterior and inferior to portions of the root.

Arch of the Aorta

The arch of the aorta curves over the superior aspect of the left lung root.

This relationship is comparable, as a gross topographical landmark, to the arch of the azygos vein passing above the right lung root.

Descending Thoracic Aorta

The descending thoracic aorta lies posterior to the left lung root and contributes to the characteristic impressions on the mediastinal surface of the left lung.

Its close relationship is important in thoracic imaging and surgical anatomy.

Root Movements During Respiration

The lungs change position and dimensions during breathing, and structures entering the lungs must accommodate these movements.

The pleural sleeve and pulmonary ligament permit a degree of movement and reduce restriction of the root structures as the lungs expand and recoil.

Bronchovascular Branching

After entering the lung, the bronchi and pulmonary arteries divide progressively and generally remain associated as they travel toward bronchopulmonary segments.

Pulmonary veins differ by tending to occupy intersegmental planes rather than accompanying bronchi centrally within individual segments.

Imaging of the Lung Root

The structures of the lung roots contribute to the hilar appearances seen on chest radiographs and computed tomography.

Pulmonary arteries and their branches account for much of the normal hilar vascular shadow, while CT allows individual bronchi, arteries, veins, lymph nodes, and surrounding structures to be distinguished more precisely.

Root Enlargement

Apparent enlargement of the root or hilar region can result from enlarged pulmonary vessels, lymphadenopathy, tumors, or other mediastinal and pulmonary abnormalities.

Cross-sectional imaging is useful for identifying which component is responsible for the abnormal appearance.

Clinical Importance of Lymph Nodes

Bronchopulmonary lymph nodes are clinically important because pulmonary infection, inflammatory disease, and malignant cells can spread through lymphatic pathways toward the lung root.

Assessment of hilar and mediastinal lymph nodes is therefore an important component of thoracic imaging and lung cancer staging.

Lung Cancer

Central lung tumors may arise near or extend toward the lung root, where they can involve bronchi, pulmonary vessels, lymph nodes, and adjacent mediastinal structures.

The anatomical relationship of a tumor to these structures can influence symptoms, staging, surgical planning, and resectability.

Pneumonectomy

A pneumonectomy involves removal of an entire lung and requires surgical division and secure management of the major structures forming the lung root.

Precise knowledge of the side-specific arrangement of pulmonary arteries, veins, and bronchi is essential during this procedure.

Lobectomy

During a lobectomy, branches of the pulmonary artery, pulmonary veins, and lobar bronchi supplying the selected lobe must be identified and divided while preserving structures supplying the remaining lung.

The organization of the root therefore provides the proximal anatomical basis for lobar pulmonary surgery.

Lung Transplantation

Lung transplantation requires surgical connections involving the bronchus, pulmonary artery, and pulmonary venous drainage of the donor lung.

Understanding the anatomy of the lung root is fundamental to planning and performing these vascular and airway anastomoses.

Functional Components of the Root

Component	Primary Role
Bronchi	Conduct air between the trachea and pulmonary tissue
Pulmonary arteries	Carry deoxygenated blood to the lungs
Pulmonary veins	Return oxygenated blood to the left atrium
Bronchial vessels	Supply and drain supporting pulmonary tissues
Lymphatics	Drain lymph from the lungs
Autonomic nerves	Regulate airway, vascular, and glandular functions

Key Anatomical Relationships

• The root connects each lung to the mediastinum.
• The root passes through the hilum on the mediastinal surface of the lung.
• Its contents include bronchi, pulmonary vessels, bronchial vessels, lymphatics, nerves, and connective tissue.
• The right superior lobar bronchus is eparterial.
• The right pulmonary artery is generally anterior to the bronchus.
• The left pulmonary artery is generally superior to the bronchus.
• Pulmonary veins occupy relatively anterior and inferior positions.
• The inferior pulmonary vein is the lowest major structure in each root.
• The phrenic nerve passes anterior to each root.
• The vagus nerve passes posterior to each root.
• The azygos vein arches above the right root.
• The aortic arch passes above the left root.
• The descending thoracic aorta lies posterior to the left root.
• A pleural sleeve surrounds the root near the lung.
• The pulmonary ligament extends inferiorly from this pleural reflection.

Clinical Importance

The root of the lung is a compact anatomical region containing the major structures responsible for ventilation, pulmonary circulation, systemic bronchial circulation, lymphatic drainage, and autonomic innervation of the lung. Its anatomy is therefore fundamental to understanding the organization of the respiratory system.

The arrangement differs significantly between the right and left sides. The eparterial bronchus is a distinctive feature of the right root, while the left pulmonary artery occupies the highest major position at the left root. The phrenic nerves pass anterior to both roots, whereas the vagus nerves pass posteriorly.

Understanding the root of the lung provides the anatomical foundation for studying the pulmonary hilum, main and lobar bronchi, pulmonary arteries, pulmonary veins, bronchial vessels, pulmonary plexuses, bronchopulmonary lymph nodes, pulmonary ligament, thoracic imaging, and pulmonary surgery.