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Azygos vein

The azygos vein (Latin: vena azygos) is a venous blood vessel that ascends along the right side of the vertebral column. It connects the systems of the superior vena cava (SVC) and inferior vena cava (IVC), providing an alternative pathway for venous blood to reach the right atrium of the heart if either of the major veins is blocked. The main tributaries of the azygos vein are the hemiazygos vein and the accessory hemiazygos vein.


Course of azygos vein

The course of the azygos vein usually begins behind the IVC at the level of the renal veins. It usually originates in the lumbar vertebral level but may start further up. The azygos vein ascends and reaches the thoracic cavity by passing behind the right crus or the aortic hiatus if the diaphragm. It travels upward within the posterior mediastinum, usually lying to the right of the thoracic duct. The vein reaches the level of the 4th thoracic vertebra (T4) and then arches above the right pulmonary hilum. The azygos vein empties into the superior vena cava just before the SVC pierces the pericardial sac.


Relations with other structures

The following structures are located just posterior to the azygos vein: 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: thoracic duct, aorta, esophagus, trachea, as well as the right vagus nerve (CN X). And laterally to the azygos vein are located the following structures: right greater splanchnic nerve, lung, and pleura.