Azygous vein drains into:

The origin of the azygos vein is variable. It is often formed by the union of the right ascending lumbar vein and the right subcostal vein.It ascends through the aoic opening in the diaphragm on the right side of the aoa to the level of the fifth thoracic veebra.Here it arches forward above the root of the right lung to empty into the posterior surface of the superior vena cava. The azygos vein has numerous tributaries, including the fifth to eleventh right posterior intercostal veins, the right superior intercostal vein, the hemiazygos and the accessory hemiazygos veins, and numerous esophagral, mediastinal and pericardial veins.