An unpaired vein that arises from the inferior vena cava and drains into the superior vena cava, returning blood from the thorax and abdominal cavities.
A single vein arising in the abdomen as a branch of the ascending lumbar vein. It passes upward through the aortic hiatus of the diaphragm into the thorax, then along the right side of the vertebral column to the level of the fourth thoracic vertebra, where it turns and enters the superior vena cava. In the thorax, it receives the hemiazygos, accessory azygos, and bronchial veins, as well as the right intercostal and subcostal veins. If the inferior vena cava is obstructed, the azygos vein is the principal vein by which blood can return to the heart.