The azygos vein (Latin: vena azygos) is a vessel that ascends along the right side of the vertebral column. It connects the systems of the superior vena cava and inferior vena cava, providing an alternative pathway for venous blood to reach the right atrium if either of the major veins is blocked.
The main tributaries of the azygos vein are the hemiazygos vein and the accessory hemiazygos vein.
The azygos vein usually begins from the posterior aspect of the inferior vena cava at the level of the renal veins. It usually originates in the lumbar level but may start further up.
This vein ascends and reaches the thoracic cavity by passing behind the right crus of the diaphragm or via the aortic hiatus. It travels upward within the posterior mediastinum, usually lying to the right of the thoracic duct. It reaches the level of the vertebra T4 and then arches above the right pulmonary hilum. The azygos vein empties into the superior vena cava just before it pierces the pericardial sac.
The following structures are located just posterior to the azygos vein: the anterior longitudinal ligament, right posterior intercostal arteries, and bodies of vertebrae T4-T2.
In the posterior mediastinum, the recess of the right pleural sac and the esophagus are located anterior to the azygos vein.
Medial to the vein, the following structures can be found: the thoracic duct, aorta, esophagus, trachea, as well as the right vagus nerve.
And laterally to the azygos vein are located the following structures: the right greater splanchnic nerve, lung, and pleura.