Introduction: the azygos system consists of veins on each side of the vertebra column that drains the back and thoracic and abdominal walls. It Is variable in origin, course, tributaries, anastomosis and termination.
1. The azygos vein
Origin: it arises from the posterior aspect of the inferior vena cava.
Course: it ascends in the posterior mediastinum close to the right side of the bodies of the inferior eight thoracic vertebrae. It is covered anteriorly by the esophagus as it passes posterior to the root of the right lung. It then joins the superior vena cava at the level of the fourth thoracic vertebra (T4)
Communication: the azygos vein communicates with the posterior intercostal veins and the vertebral venous plexus.
2. Hemiazygos vein:
Origin: this arises on the left side by the junction of the left subcostal and ascending lumbar veins.
Course: it ascends on the left side of the vertebral column posterior to the thoracic aorta as far as the ninth thoracic vertr. It crosses to the right and joins the azygos.
Tributaries: it receives the inferior four posterior intercostal veins, the inferior esophageal veins and small mediastinal veins.
3. Accessory hemiazygos vein:
Origin: this vein begins at the medial end of the 4th or 5th intercostal space
Course: it descends on the left side of the vertebral column from T5 to T8. It crosses to the right side and joins the azygos vein
Tributaries: it receives tributaries from veins in the fourth to eight intercostal spaces.
Function: the azygos system of veins provides an alternate route of venous drainage when the inferior vena cava is obstructed.