ArticlesAnatomyThoraxLungsHilum of the lung

ArticlesAnatomyThoraxLungsHilum of the lung

Hilum of the lung

Author: Alice Ferng, B.S., MD, PhD • Reviewer: Dimitrios Mytilinaios, MD, PhD
Last reviewed: October 30, 2023
Reading time: 7 minutes

Each lung (the right and left lungs) can be divided into four main sections: the apex, base, root, and the hilus, or hilum of the lung. Hila, or lung roots, are relatively complicated structures that consist mainly of the major bronchi and the pulmonary arteries and veins.

The hilum of the lung is found on the medial aspect of each lung, and it is the only site of entrance or exit of structures associated with the lungs. That is to say, both lungs have a region called the hilum, which serves as the point of attachment between the lung root and the lung. Broadly speaking, this particular region of the lung can be described as a triangular, depressed area where a lot of anatomical structures enter and leave each lung.

Worried about learning all of the structures of the lungs and respiratory system? Ease into this topic at your own pace with our respiratory system quizzes and labeled diagrams.

This article will analyze the hilum in detail, providing you with its exact location, the structures that one can find there, together with some clinical aspects.

Key facts about the root of the lung
Relations and borders	Anterior - phrenic nerve, anterior pulmonary plexus, superior vena cava, part of right atrium (right lung) Posterior - vagus nerve, posterior pulmonary plexus, descending aorta (left lung only) Inferior - pulmonary ligament Superior - azygos vein, aortic arch
Hilum location and relations	Mediastinal surface of the lung, cardiac impression, pleura (visceral and parietal)
Contents	Principal bronchus, lobar bronchi (superior, middle, inferior), one pulmonary artery, two pulmonary veins, bronchial arteries and veins, pulmonary nervous plexus, lymphatics, pulmonary lymph nodes, areolar tissue
Clinical aspects	Bilateral and symmetrical hilar enlargement (sarcoidosis) Asymmetric hilar enlargement (cancer) Change in position (hemo-/pneumothorax)

Contents

Hilum location
Lung roots
Clinical notes
1. Changes in size/density
2. Changes in hilar position
Sources

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Hilum location

Structurally, the hilum is a large triangular depressed area on the lung that is located just superior to the center of the mediastinal surface and behind the cardiac impression of each lung, and is found nearer to the back border than to the front. The rib cage is separated from the lung by a two-layered membranous coating called the pleura. The hilum is where the connection between the parietal pleura (covering the rib cage) and the visceral pleura (covering the lung) connect, which denotes the meeting point between the mediastinum and the pleural cavities.

Lung roots

Lung roots are enclosed in a short tubular sheet of pleura that joins the pulmonary and mediastinal parts of pleura. The lung root extends inferiorly as a narrow fold known as the pulmonary ligament. Additionally, it lies opposite to the bodies of the fifth, sixth, and seventh thoracic vertebrae. Structures that form the root of the lung enter and exit at the hilum, and allow the root to be connected to the heart and to the trachea. Functionally, this means that the hilum aids the lung roots by anchoring the lungs to the heart, trachea, and surrounding structures.

The lung root is formed by: the bronchus, the pulmonary artery and veins, the bronchial arteries and veins, the pulmonary plexuses of nerves, lymphatic vessels, bronchial lymph glands, and areolar tissue, all of which are enclosed by a layer of the pleura, which is a thin smooth layer of protective tissue.

More specifically, a lung root has:

a principal bronchus on one side
the eparterial and hyparterial bronchus on the other side
one pulmonary artery
two pulmonary veins (superior and inferior)
bronchial arteries (one on one side, and two on the other side)
bronchial veins
anterior and posterior pulmonary nerve plexuses
lymphatics
bronchopulmonay lymph nodes
areolar tissue

Medial view and hilum of cadaveric lungs: The lung hilum contains the main bronchus, pulmonary artery and pulmonary veins. The main bronchus is located posterior to the vessels, while the pulmonary artery is superior to the vein. In addition, the pulmonary arteries have thicker walls compared to veins, while the bronchus contains cartilage in its walls. Look out for these distinguishing features upon your next inspection of the lungs.

Catch up on the general anatomy of the lungs with this integrated quiz.

Anatomically, the root of the right lung lies behind the superior vena cava and part of the right atrium, and beneath the azygos vein. The root of the left lung passes under the aortic arch and in front of the descending aorta. Other structures include the phrenic nerve, pericardiacophrenic artery and vein, and the anterior pulmonary plexus, which lie in front of each lung root, whereas the vagus nerve and posterior pulmonary plexus lies behind each lung root. Finally, the area surrounding the hilum of the lung is called the perihilar region.

Quiz yourself on the structures found at the hilum of the lung or take a look at our study unit:

Clinical notes

Hilar abnormalities can present both unilaterally and bilaterally. Since the hila consist of vessels, bronchi, and lymph nodes, changes in a chest X-ray will present as a change in position, size, and/or density of the hilar region. Common causes of these changes will include various types of lung cancers.

Changes in size/density

Bilateral, symmetrical hilar enlargement should raise the suspicion of sarcoidosis, especially in the presence of paratracheal enlargement, or lung parachymal shadowing. Clinically, the patient often presents with joint pain and erythema nodosum. Differentials include pulmonary arterial hypertension, which may also cause bilateral and symmetrical hilar englargement, or lymphoma, metastatic disease, or infection.

Asymmetric hilar enlargement is often the result of breast

Changes in hilar position

Abnormal hilar position should be approached by looking at whether the structure has been pushed or pulled. Is there a lung abnormality that has reduced or increased the volume of one lung (such as by hemothorax or pneumothrorax)? Does the patient have a past medical history that would have caused a chance in position? Such as having a hilar malignancy that is treated with radiotherapy, and has resulted in a deviation from normal hilar position.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References:

Chest x-ray anatomy: Hilar structures. Radiology Masterclass 2007. (accessed 18th of August 2014)
Chest x-ray anatomy: Hilar abnormalities. Radiology Masterclass 2007. (accessed 18th of August 2014)

Illustrators:

Pulmonary ligament (medial view) - Yousun Koh
Medial view of the lung - Yousun Koh
Medial view and hilum of cadaveric lungs - Prof. Carlos Suárez-Quian

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