Gross anatomy of Azygos venous system : Azygos vein, Accessory hemiazygos vein and Hemiazygos vein

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Azygos venous system

Gross anatomy

Azygos vein
The azygos vein initially arises at the union of the right ascending lumbar vein and the right subcostal vein around T12. It enters the thorax via the aortic hiatus in the diaphragm and ascends posteriorly alongside the vertebral bodies, arching over the right main bronchus at T5-T6 and enters the superior vena cava (SVC) at T4 2.

The azygos vein receives the lower eight posterior intercostal veins, bronchial veins from the right lung and is joined by the right superior intercostal vein superiorly. Although there is much variability, the azygos vein receives the accessory hemiazygos vein and the hemiazygos vein at the level of T8 and T9 respectively.

The azygos vein contains a valve halfway along the azygos arch (between the vertical azygos vein and the point where the azygos vein enters the SVC). This valve is a common place for the intravenously injected contrast agent to accumulate, which may simulate pathology on CT 3.

Accessory hemiazygos vein
The accessory hemiazygos vein drains the superior left hemithorax. It arises from the 4th to 8th left posterior intercostal veins and lies longitudinally on the left side of the vertebral bodies. It also drains the left bronchial vein and some veins from the esophagus. It joins the azygos vein behind the esophagus at the level of T8.

Hemiazygos vein
The hemiazygos vein usually arises from the left ascending lumbar vein and passes through the left crus of the diaphragm. It drains the 9th to 11th left posterior intercostal veins and left subcostal vein, alongside some esophageal veins. The hemiazygos vein ascends the thorax to the left of the vertebral bodies and passes behind the esophagus at the level of T9 to join the azygos vein.

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