The azygos vein usually begins from the posterior aspect of the inferior vena cava (read more!) at the level of the renal veins. It usually arises at the level of the lumbar vertebrae, but may originate further up.

The vein ascends and reaches the thoracic cavity passing behind the right crus of the diaphragm. Alternatively, it can reach the thoracic cavity via the aortic hiatus of the diaphragm. The azygos vein ascends within the posterior mediastinum. It usually goes to the right of the thoracic duct (read more!). At the level of the fourth thorcic vertebra, the azygos vein arches above the right pulmonary hilum and drains into the superior vena cava (read more!) just before it pierces the pericardial sac.

Posterior to the azygos vein lie the following structures - anterior longitudinal ligament, right posterior intercostal arteries and bodies of the vertebrae T4 - L2.
In the posterior part of the thorax, anterior to the azygos vein is located the recess of the right pleural sac and esophagus.
Medial to the vein are the following structures - thoracic duct, thoracic aorta, esophagus, trachea and right vagus nerve (CN X).
Lateral to the azygos vein are found the following structures - right greater splanchnic nerve, lungs and pleura.

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Azygos vein