Corpus: Azygos vein

The azygos vein is the right-sided continuation of the ascending lumbar vein, extending cranially above the diaphragm. It drains venous blood into the superior caval vein. It is a key component of the azygos venous system.

The anatomy of the azygos vein is highly variable. It runs along the right side of the thoracic vertebrae and passes through the right medial lumbar fissure of the diaphragm, along with the greater splanchnic nerve, from the abdominal cavity into the thoracic cavity.

In the thorax, the azygos vein ascends through the posterior mediastinum, reaching the level of the 4th or 5th thoracic vertebra. At this point, it bends anteriorly, arches over the right main bronchus, and empties into the superior caval vein from the medial-dorsal side, just outside the pericardium. The arch of the azygos vein serves as an important anatomical landmark.

The azygos vein receives blood from several tributaries during its course:

• Posterior intercostal veins, including intervertebral veins
• Right superior intercostal vein
• Esophageal veins
• Bronchial veins
• Hemiazygos vein: typically joins at the level of the 7th-10th thoracic vertebrae

The azygos vein develops from the right supracardinal vein during embryogenesis.

The azygos and hemiazygos veins are connected to the inferior caval vein via the ascending lumbar veins and the lumbar veins, forming an important collateral circulation between the inferior and superior caval vein. If there is an obstruction in the inferior caval vein, venous blood can bypass the blockage and reach the superior caval vein via the azygos system.

Azygos continuity syndrome is a rare congenital anomaly in which part of the inferior caval vein fails to develop, and blood is diverted through the azygos vein to the superior caval vein.

Agenesis of the azygos vein is an extremely rare condition. When it occurs, the hemiazygos and accessory hemiazygos veins take over the drainage of the intercostal veins on both sides of the thorax.