What is the difference between hilum of the lung and root of the lung?

What is the difference between hilum of the lung and root of the lung?

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In my book they describe root of the lung like this:
In the region of vertebrae TV to TVII, the mediastinal pleura refects off the mediastinum as a tubular, sleeve-like covering for structures (i.e., airway, vessels, nerves, lym-phatics) that pass between the lung and mediastinum. This sleeve-like covering, and the structures it contains, forms the root of the lung.

And hilum of the lung like this: The root joins the medial surface of the lung at an area referred to as the hilum of the lung. Here, the mediastinal pleura is continuous with the visceral pleura.

I dont see the difference between Root and hilum. Can somebody please help me out?

Short answer: Hilum is an area(has only 2 dimensions) where as root is a body( has 3 dimensions).

The root of lung is a short broad pedicle connecting the medial surface of the lung with the mediastinum. It consists of structures entering and leaving the lung at hilum. The root of lung is surrounded by a tubular sheath derived from the mediastinal pleura.


The hilum is the area on the mediastinal surface of the lung through which structures enter or leave the lung.

Source: pg no 239

Consider the following image( edited by myself):

If the cylinder is root of lung then lateral opening(blue) is hilum.

The region bounded( within the limits of pleural membrane) by those two planes is root of lung and the area bounded by the lateral plane is hilum.

The root of the lung is region of the mediastinal surface of the lung that lies between T5 and T7, it consist of the mediastinal and visceral pleura reflecting over the structures entering or leaving the lungs at the hilum like a sleeve.

What is the difference between hilum of the lung and root of the lung? - Biology

By the end of this section, you will be able to:

  • Describe the overall function of the lung
  • Summarize the blood flow pattern associated with the lungs
  • Outline the anatomy of the blood supply to the lungs
  • Describe the pleura of the lungs and their function

A major organ of the respiratory system, each lung houses structures of both the conducting and respiratory zones. The main function of the lungs is to perform the exchange of oxygen and carbon dioxide with air from the atmosphere. To this end, the lungs exchange respiratory gases across a very large epithelial surface area—about 70 square meters—that is highly permeable to gases.

Anatomy of the Hilum

Both the right and the left lung have a hilum which lies roughly midway down the lungs, and slightly towards the back (closer to the vertebrae than to the front of the chest). Each lung may be visualized as having an apex (the top), a base (the bottom), a root, and a hilum.

The major bronchi, pulmonary arteries, pulmonary veins, and nerves are the structures which enter and exit the lungs in this region. Lymph nodes, called hilar lymph nodes, are also present in this region. Both hilum are similar in size, with the left hilum usually found slightly higher in the chest than the right hilum.

Arrangement of Structures In The Root Of The Lung At The Hilum

The arrangement of structures in the roots of the lungs is as follows:

1. From before backwards (it’s more or less quite similar on 2 sides):

  • Pulmonary vein (superior).
  • Pulmonary artery.
  • Bronchus (left main bronchus on the left side, and eparterial, and hyparterial bronchus on the right side).

Mnemonic: VAB (Vein, Artery, and Bronchus).

2. From above downwards (it differs on 2 sides):.

Right side
Left side
• Eparterial bronchusPulmonary artery
• Pulmonary arteryLeft principal bronchus
• Hyparterial bronchusInferior pulmonary vein
• Inferior pulmonary vein

The difference in the arrangement of structures from above downwards on the 2 sides is because right main bronchus before going into the lung at hilum splits into 2 lobar bronchi, the upper lobar bronchus enters above the pulmonary artery (eparterial bronchus) and lower lobar bronchus enters below the pulmonary artery (hyparterial bronchus).


The lungs are our two respiratory organs. They are laterally placed within the pleural cavities of the thorax. The bronchial tree conducts air into and out of the lungs.

At their most basic level, each lung is characterized by its apex and base the apex projects towards the superior thoracic aperture, while the base is placed on the diaphragm. Alternatively, we can describe the lung as having three surfaces (costal, medial and diaphragmatic) which are divided by three borders (anterior, posterior and inferior). The medial surface, located between the anterior and posterior borders, can be further subdivided into the vertebral and mediastinal surfaces of the lung. The mediastinal surface is of interest because it contains the lung hilum. The hilum of the lung is a passage for the pulmonary artery, two pulmonary veins and the main bronchus, as well as bronchial arteries and veins, nerves and lymphatic vessels.

The two lungs are not a mirror reflection of one another. The right lung has three lobes inferior, superior and middle. These lobes are further divided, giving 10 bronchopulmonary segments, which are the functional units of the lung tissue. The right lung lobes are separated by two fissures oblique and horizontal. The mediastinal surface of the right lung is in contact with the heart, superior vena cava, inferior vena cava, azygos vein and the esophagus. The impressions of these structures can be seen on the medial lung surface.

Lung in a cadaver: During your dissection sections, you will be asked to place the lungs in the correct anatomical position, like in situ. The landmarks that can guide you are the hilum (medially), the apex (superiorly) or the concave base (inferiorly) and the anterior border (anteriorly). The anterior border might seem quite difficult to distinguish, but in fact, it has a sharp and fine aspect upon palpation, making it easily identifiable.

On the other hand, the left lung only has two lobes superior and inferior, and 8 lung segments. The lobes are separated by a single oblique fissure. The mediastinal surface of the left lung shows impressions of the following structures: the heart, aortic arch, thoracic aorta and esophagus. Both the left and right lung have interlobar surfaces which are, as their name suggests, the surfaces of lobes facing one into another separated with fissure.

Curious to find out everything about the lung anatomy? Sure, we have an additional resource for you.

Lung innervation

The lungs and visceral pleura are supplied by the anterior pulmonary plexus and posterior pulmonary plexus which, as indicated by their names, are placed anteriorly and posteriorly to the tracheal bifurcation.

The sympathetic source for the plexus is the sympathetic trunk, while the parasympathetic source is the vagus nerve. They act in synchronization, with sympathetic stimulation leading to bronchial dilation, and parasympathetic leading to constriction of the bronchi.

Lymphatic drainage of lungs

The lymph of the lungs drains into the tracheobronchial nodes which are placed around the main and lobar bronchi, and along the sides of trachea. They extend from the inside of the lung through the hilum and posterior mediastinum. The vessels from the tracheobronchial nodes unite with vessels from the parasternal and brachiocephalic nodes, forming the left and right bronchomediastinal trunks. These trunks eventually drain either into the venous angle or thoracic duct.

Solidify your knowledge about the lymphatic drainage of the lungs with this study unit.

Chest X-ray Anatomy Hilar structures

The lung roots, or hila (singular – hilum), are complicated anatomical structures containing the pulmonary vessels and the major bronchi, arranged asymmetrically.

Although the hilar lymph nodes are not visible on a normal chest X-ray , they are of particular importance clinically. Often, hilar enlargement is due to enlargement of these nodes.

Many disease processes involve the hila, so a knowledge of their normal X-ray appearance is important. Both hila should be of similar size and density. If either hilum is bigger and more dense (whiter) than normal, this may indicate an abnormality.

Normal hilar position

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Normal hilar position

  • Each hilar point is the angle formed where the upper and lower lobe pulmonary vessels meet
  • They are useful points of reference to determine the position of the hila
  • Commonly the left hilum is higher than the right
  • Note: Not every normal patient has a clearly-defined hilar point on both sides

Pulmonary arteries

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Pulmonary arteries

  • Deoxygenated blood (blue arrows) is pumped upwards out of the right ventricle (RV) via the main pulmonary artery (Main PA)
  • The main PA divides into left pulmonary artery (Left PA) and right pulmonary artery (Right PA) which pass into the lungs via the hila
  • The left PA hooks backwards over the left main bronchus
  • The right PA passes anterior to the right main bronchus

Assessing the hila

Chest X-ray assessment routinely involves checking the hilar structures for normal, size, density and position. The hila are often wrongly called abnormal when normal and normal when abnormal. An awareness of the range of normal is important, but the best tip is to look for increase in density as well as size. If the hila are out of position, ask yourself if they are pushed or pulled, just as you would when assessing the trachea.

Page author: Dr Graham Lloyd-Jones BA MBBS MRCP FRCR - Consultant Radiologist - Salisbury NHS Foundation Trust UK ( Read bio )

Chest X-ray Abnormalities Hilar abnormalities

The hila consist of vessels, bronchi and lymph nodes. On a chest X-ray, abnormalities of these structures are represented by a change in position, size and/or density.

Hilar enlargement

Hilar enlargement may be unilateral or bilateral, symmetrical or asymmetrical. In combination with clinical information, each of these patterns is often helpful in reaching a diagnosis. Bilateral, symmetrical hilar enlargement should raise the suspicion of sarcoidosis, particularly if there is evidence of paratracheal enlargement, or lung parenchymal shadowing.

Bilateral hilar enlargement

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Bilateral hilar enlargement

Clinical information



  • Lymphoma, metastatic disease or infection may occasionally look similar
  • Pulmonary arterial hypertension may also cause bilateral symmetrical hilar enlargement

Asymmetric hilar enlargement

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Asymmetric hilar enlargement

  • Both hila are larger and denser than normal
  • The right hilum is bigger than the left
  • Multiple small lung nodules
  • Missing right breast shadow (mastectomy)

Clinical information


Hilar position

If a hilum has moved, you should try to determine if it has been pushed or pulled, just like you would for the trachea. Ask yourself if there is a lung abnormality that has reduced volume of one hemithorax (pulled), or if there has been increase in volume or pressure of the other hemithorax (pushed).

Abnormal hilar position

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Abnormal hilar position

  • The left hilum is large, dense and pulled laterally and upwards to the left
  • The trachea is deviated (pulled) towards the left, indicating loss of lung volume in the left hemithorax

Clinical details


Page author: Dr Graham Lloyd-Jones BA MBBS MRCP FRCR - Consultant Radiologist - Salisbury NHS Foundation Trust UK ( Read bio )

Last reviewed: July 2019

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Parts of the lungs

Right Lung

This part of the lung has three lobes and has more segments than the left. These lobes are divided into superior (upper), middle, and inferior (lower), by horizontal and oblique fissures. The right lung has a deep concavity on the inner surface that is called the cardiac impression, at the same level as the heart. It also has two bronchi, airway passages that regulate the flow of air into the lungs.

Regarding its size, it is shorter and wider than the left lung.

Left Lung

Unlike the right lung, the left lung has only two lobes instead of three, the upper and the lower. It is divided by a deep oblique fissure. With the heart close to the left lung, it partly compresses the anterior border of the left lung and so makes it comparatively smaller than the right lung. While it does not have a middle lobe like the right lung, it does have its ‘lingula’ (little tongue), a tongue-like projection on the superior lobe.

Regarding its size, even though it is a bit longer, it is still smaller than the right lung because of the heart occupying space.

Blood Supply to the Lungs

Pulmonary circulation transports oxygen-depleted blood away from the heart to the lungs and returns oxygenated blood back to the heart.

Learning Objectives

Distinguish between pulmonary and systemic circulation of blood

Key Takeaways

Key Points

  • Pulmonary circulation is the portion of the cardiovascular system that carries oxygen-depleted blood to the lungs from the heart, and returns oxygenated (oxygen-rich) blood back to the heart.
  • Systemic circulation is the portion of the cardiovascular system that brings oxygen to the tissues of the body. De-oxygenated blood enters the right atrium. Blood then moves to the right ventricle that pumps blood from the heart to the lungs where it releases carbon dioxide and picks up oxygen.
  • Oxygenated blood leaves the lungs through pulmonary veins, completing the pulmonary cycle. This blood enters the left atrium and is then transferred to the left ventricle, which pumps the newly oxygenated blood back into systemic circulation.
  • The pulmonary arteries carry deoxygenated blood to the lungs, where it releases carbon dioxide and picks up oxygen during respiration.
  • A pulmonary embolism can occur if blood pools in veins of the legs and forms a blood clot due to immobilization. The resulting blood clot can block off the pulmonary artery and cause the alveoli inside the lung to die.

Key Terms

  • pulmonary embolism: A blockage of the blood supply the lungs by a blood clot.
  • atrium: An upper chamber of the heart that receives blood from the veins and forces it into a ventricle.
  • ventricle: The lower chamber of the heart that receives blood from the atrium and pumps it into the arteries.

There are two primary types of circulation in the human.

  1. Pulmonary circulation refers to blood supply to the lungs for the purpose of gas exchange.
  2. Systemic circulation refers to blood supply to the rest of the body, for the purpose of supplying oxygen to the tissues.

Bronchial circulation (by the bronchial arteries) supplies blood to the tissues of the bronchi and the pleura, and is considered part of systemic circulation.

Pulmonary Circulation

The right side of the heart deals with pulmonary circulation. At the end of systemic circulation, the veins take blood back to the heart through the vena cava.

The vena cava fills the right atrium with blood, which then ejects blood into the right ventricle by passing through the tricuspid valve. After blood fills in the right ventricle, it contracts and pumps the blood through the pulmonary valve, and into the pulmonary arteries.

There are two pulmonary arteries (one for each lung) that bring the deoxygenated blood to the lungs through the hilium. The arteries branch into the capillaries of the alveoli. Capillaries are the thinnest and smallest type of blood vessel, and they supply oxygen to individual tissues everywhere in the human body.

Gas exchange occurs by passive diffusion in the alveoli, so that dissolved oxygen enters the capillaries, while carbon dioxide leaves pulmonary circulation. The oxygenated blood then leaves the lungs through pulmonary veins (also contained in the hilium), which return the blood to the left side of the heart, completing the cycle of pulmonary circulation.

This blood then enters and fills inside the left atrium, which pumps it through the mitral valve (also called bicuspid) into the left ventricle. The blood fills inside the left ventricle and is then pumped through the aortic valve into the aorta, which marks the beginning of systemic circulation.

Systemic circulation and pulmonary circulation form the overall cycle of the circulatory system: transporting oxygen throughout the body.

Pulmonary circuit: Diagram of pulmonary circulation. Oxygen-rich blood is shown in red oxygen-depleted blood in blue.

Problems in Pulmonary Circulation

While the cycle of pulmonary and systemic circulation is a well designed and effective system, it is not immune to certain problems. The most serious issue in pulmonary circulation is a pulmonary embolism, which is where a blood clot travels to the lung and causes an infarction of the lung (tissue death from lack of oxygen).

These blood clots typically originate in the deep veins of the legs (part of systemic circulation) as a result of blood pooling from injury or immobilization. As the veins of the leg are on their way to the right side of the heart, the clots are less likely to break up before they reach pulmonary circulation.

When the clot reaches the pulmonary artery, it obstructs the flow of blood into the lung, which causes the alveoli in the effected lung to die as a result. This results in an increase in aveolar dead space and decreased perfusion, (leading to shortness of breath and chest pain) and can be fatal if not treated in time by fibrinolytics (medications that dissolve the clot).

Watch the video: Heilung. LIFA - Krigsgaldr LIVE (February 2023).