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Azygos vein

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Contents of the mediastinum seen from the lateral views.
Azygos vein (Vena azygos)

The azygos venous system is located on either side of the vertebral column and drains the viscera within the mediastinum, as well as the back and thoracoabdominal walls. This system consists of the azygos vein and its two main tributaries: the hemiazygos vein and the accessory hemiazygos vein.

The azygos vein usually arises either from the lumbar azygos vein or from the posterior side of inferior vena cava around the level of renal veins. It passes through the diaphragm, reaches the mediastinum and finally drains into the superior vena cava. The azygos venous system has many anastomoses with inferior vena cava and vertebral venous plexuses. This makes it an important connection between the two venae cavae which allows alternate drainage route in case of inferior vena cava obstruction,

This article will discuss the anatomy and function of the azygos vein.

Key facts about the azygos vein
Origin Lumbar azygos vein/ inferior vena cava
Tributaries Hemiazygos vein, accessory hemiazygos vein
Drains to Superior vena cava
  1. Anatomical course
  2. Anatomical relations
  3. Tributaries
  4. Anatomical variations
  5. Clinical notes
    1. Laceration
    2. Aneurysm
    3. Congenital defects
  6. Sources
+ Show all

Anatomical course

The azygos vein usually originates from the posterior aspect of the inferior vena cava, at the level of the renal veins. It ascends within the posterior mediastinum to the level of T4 before it arches above the right pulmonary hilum. It drains into the superior vena cava just before it pierces the pericardium.

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Anatomical relations

Relationships of the azygos vein to other structures within the mediastinum can be important in clinical practice. The following structures are located posterior to the azygos vein:

In the posterior thorax, the azygos vein is located posterior to the recess of the right pleural sac and the oesophagus.

M​edial to the vein one can find:

  • the thoracic duct
  • aorta
  • oesophagus
  • trachea
  • right vagus nerve

The following structures are located laterally to the axygos vein:

  • the right greater splanchnic nerve
  • lung
  • pleura

The azygos vein is also in close proximity to the right posterolateral aspect of the descending thoracic aorta. Pulsations within the aorta may aid venous return in the azygos vein, as well as the hemiazygous vein.

Master the relations between the structures of the mediastinum with our articles, video tutorials, quizzes, and labelled diagrams.


The azygos vein has two tributaries, which are referred to as the hemiazygos vein and the accessory hemiazygos vein.

The hemiazygos vein is often connected to the left renal vein. It is formed by the oesophageal and mediastinal tributaries, the common trunk of the left ascending lumbar vein and left subcostal vein, and by the lower three posterior intercostal veins. It ascends anterior to the vertebral column before crossing the column posterior to the aorta, oesophagus and thoracic duct at the level of T8.

The accessory hemiazygos vein is formed by veins from the fourth to eighth intercostal spaces and sometimes by the left bronchial veins. It descends to the left of the vertebral column before crossing T7, where it joins with the azygos vein. Sometimes it joins the hemiazygos vein and, in this case, their common trunk drains into the azygos vein.

Anatomical variations

The origin and anatomical course of the azygos vein are quite variable. Usually, there is a singular azygos vein on the right side of the body. However, the azygos vein is occasionally located in the midline or two independent veins may be present like in early embryonic development.

The azygos vein usually originates at the level of the lumbar vertebrae, but may originate further up. The lumbar aspect of the azygos vein can ascend anterior to the lumbar vertebrae and pass behind the right crus of the diaphragm or cross the aortic hiatus (where the aorta pierces the diaphragm) to the right of the cisterna chyli, a dilated lymph sac.

The common trunk of the right ascending lumbar vein and the right subcostal vein joins the azygos vein anterior to the body of T12. However, if the lumbar segment is absent, this trunk may form the azygos vein.

Feeling ready to test your knowledge about the neurovasculature of the posterior mediastinum? Try out our quiz below:

Azygos vein: want to learn more about it?

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