Azygos and Hemiazygos veins - Gross anatomy of Abdomen and pelvis 

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Azygos and Hemiazygos veins - Gross anatomy of Abdomen and pelvis
Azygos vein
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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 .
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 .
Accessory hemiazygos vein
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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 oesophagus. It joins the azygos vein behind the oesophagus at the level of T8.
Hemiazygos vein
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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 oesophageal veins. The hemiazygos vein ascends the thorax to the left of the vertebral bodies and passes behind the oesophagus at the level of T9 to join the azygos vein.
Development
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The azygos venous system arises from the supracardinal veins embryologically. The right supracardinal vein becomes the azygos vein, while the left supracardinal vein becomes the hemiazygos vein .
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