Ascending Lumbar Vein
Right Subcostal Vein
Right Superior Intercostal Vein
Accessory Hemiazygos Vein
Left Superior Intercostal Vein
The Azygos System drains the thoracic and abdominal walls, and provides collateral circulation when the inferior vena cava is obstructed or ligated. The Azygos vein is formed at the level of the diaphragm by junction of the Ascending Lumbar vein and the Right Subcostal vein. The azygos vein receives a number of tributaries in its thoracic portion prior to its junction with the superior vena cava. It receives direct contributions from the fifth through the eleventh right posterior intercostal veins, and those of the second through the fourth via the Right Superior Intercostal vein, which enters the azygos as it arches over the right mainstem bronchus. It usually receives the Hemiazygos and Accessory Hemiazygos veins when present. The hemiazygous vein arises, like the azygos vein, in the junction of the left subcostal and ascending lumbar veins. It receives the lower four or five posterior intercostal veins. The accessory hemiazygous occurs as a longitudinal venous channel in the upper portion of the chest on the left side. The upper part of the accessory hemiazygos vein is frequently connected with the Left Superior Intercostal vein.