Azygos Venous System

• The azygos venous system consists of longitudinal veins situated in the paravertebral region.
• Its principal components are the azygos, hemiazygos, and accessory hemiazygos veins.
• These veins are not accompanied by corresponding arteries. The term azygos is derived from Greek and means unpaired.
• The system drains venous blood from the back, thoracic wall, and abdominal wall.

Azygos Vein

• The azygos vein is located on the right side of the thoracic cavity.

Functions of Azygos Vein

1. Drains venous blood from the thoracic wall and the upper lumbar region.
2. Provides an important venous connection between the superior vena cava (SVC) and inferior vena cava (IVC).

Origin

• The azygos vein is usually formed by the union of the:
  • Right ascending lumbar vein
  • Right subcostal vein
• Occasionally, it may arise from the posterior aspect of the IVC or continue directly from the right subcostal vein.

Course

• The vein ascends and enters the thoracic cavity through the aortic opening of the diaphragm at the T12 vertebral level.
• It continues upward through the posterior mediastinum to approximately the level of the T4 vertebra.
• At T4, it arches anteriorly over the root of the right lung.
• The azygos arch then joins the superior vena cava shortly before the SVC enters the pericardium.

Termination

The azygos vein terminates in the superior vena cava, immediately before the SVC enters the pericardial sac.

Relations

• Anterior
  • Esophagus
• Posterior
  • Lower eight thoracic vertebrae
  • Right posterior intercostal arteries
• To the right
  • Right lung and pleura
  • Greater splanchnic nerve
• To the left
  • Thoracic duct
  • Descending thoracic aorta in the lower part
  • Esophagus, trachea, and right vagus nerve in the upper part

Tributaries

1. Right superior intercostal vein
2. Right posterior intercostal veins from the 5th to 11th intercostal spaces
3. Accessory hemiazygos vein
4. Hemiazygos vein
5. Right bronchial vein
6. Esophageal veins
7. Mediastinal veins
8. Pericardial veins

Hemiazygos Vein

• he hemiazygos vein, also called the inferior hemiazygos vein, represents the left-sided counterpart of the lower part of the azygos vein.

Formation

• It is formed by the union of the:
  • Left ascending lumbar vein
  • Left subcostal vein

Course

• It enters the posterior mediastinum by piercing the left crus of the diaphragm.
• The vein ascends along the left side of the vertebral column up to the T8 vertebral level.
• At T8, it turns to the right and crosses the vertebral column anteriorly.

Termination

• The hemiazygos vein terminates by joining the azygos vein at approximately the T8 vertebral level.

Tributaries

1. Left ascending lumbar vein
2. Left subcostal vein
3. Left 9th to 11th posterior intercostal veins
4. Small esophageal veins
5. Mediastinal veins

Accessory Hemiazygos Vein

• The accessory hemiazygos vein, also known as the superior hemiazygos vein, corresponds to the upper part of the azygos vein on the left side.

Formation

• It usually begins as a continuation of the left fourth posterior intercostal vein.

Course

• It begins near the medial end of the fourth intercostal space.
• The vein descends along the left side of the vertebral column.
• At about the T8 vertebral level, it turns to the right and crosses the vertebral column.

Termination

• The accessory hemiazygos vein usually terminates in the azygos vein at approximately the T8 vertebral level.

Tributaries

1. Left 5th to 8th posterior intercostal veins
2. Left bronchial vein, occasionally

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