Introduction

The hilum is located on the medial aspect of each lung and provides the only route via which other structures enter and exit the lung. The hilum also serves as the point of attachment for the lung root and is the point at which the visceral and parietal pleura connect.


Location of the Hilum

The hilum is a large triangular depression located superior to the centre of the lung’s mediastinal surface and posterior to the cardiac impression. Anteriorly, the hilum corresponds to the 4-5th costal cartilages and posteriorly to T5-T7.

 


The Root of the Lung

The root of the lung is located at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression.

Structures that form the root of the lung enter and exit at the hilum, allowing the root to be connected to the heart and trachea.

The root of the lung is formed by:

  • a principal bronchus on one side
  • the eparterial and hyparterial bronchus on the other side
  • one pulmonary artery
  • two pulmonary veins
  • the bronchial arteries (one on one side and two on the other)
  • the bronchial veins
  • the pulmonary plexuses of nerves
  • lymphatic vessels
  • bronchial lymph nodes
  • areolar tissue

All of these structures are enclosed by a reflection of the pleura. The lung root also extends inferiorly as a narrow fold known as the pulmonary ligament.

The root of the right lung lies behind the superior vena cava and part of the right atrium (beneath the azygos vein).

The root of the left lung passes beneath the aortic arch and in front of the descending aorta.

The phrenic nerve, pericardiacophrenic artery and vein, and the anterior pulmonary plexus lie in front of each lung root. The vagus nerve and posterior pulmonary plexus lie behind.

Lung root
Figure 3: Lung root structures 1

Hilar Contents and Function 

Anatomical StructureFunction
Pulmonary artery (right and left)Transports deoxygenated blood from the right ventricle to the functional airways.
Pulmonary vein (superior and inferior)Transports oxygenated blood to the left atrium.
Bronchus (right or left)Bifurcation of the trachea – allows air to transit down the respiratory tree
Pulmonary ligamentHolds the lower part of the lungs in position. Opposite the 5-7th thoracic vertebral bodies.

Clinical Relevance

Hilar appearance on chest x-ray

The hilar are visible on chest x-ray and are highlighted below. The v shape represents where the pulmonary artery from the lower lobe meets the pulmonary vein of the upper lobe – this is a key radiological landmark.

Hilum of the lung
Figure 4: A chest x-ray of the thorax in the posteroanterior view. The blue annotations on the x-ray outline the hilar of the lungs on the right and left. 2

 

Hilar abnormalities

Hilar abnormalities can be unilateral or bilateral and are most often noted on chest x-rays and CT scans.

Bilateral hilar enlargement

  • Tuberculosis
  • Sarcoidosis
  • Lymphoma
  • Pulmonary arterial hypertension
  • Metastatic malignancy

 

Hilar enlargement Sarcoidosis
Figure 5: Bilateral hilar enlargement secondary to sarcoidosis. 3

 

Unilateral hilar enlargement

  • Lung cancer
  • Metastatic malignancy (e.g. breast cancer)

 

Abnormal hilar position

  • Pneumonia – consolidation within a lung will often pull the hilar to the affected side (this can be seen on chest x-ray)
  • Pneumothorax – will often push the hilar away from the affected side (again this can be seen on chest x-ray)
Pneumothorax
Figure 6: Pneumothorax – note the large, well-demarcated area devoid of lung markings, the tracheal deviation, hilar deviation and movement of the heart away from the affected side. There is also small pleural effusion on the left side. 4

References

1. Gray H, Vandyke Carter H. Gray’s anatomy 1858. Modified by Geeky Medics.

2. Mikael Häggström. Licence: CC0. Available from: [LINK]

3. James Heilman, MD. Licence: CC BY-SA.

4. Photographed by User Clinical Cases 00:42 on 7 November 2006. Licence: CC BY-SA.


 

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