The azygos vein is so named because it is unpaired, having no matching vein on the left side of the body. The anatomy of this blood vessel can be quite variable.
It forms an anastomosis between the superior vena cava to the inferior vena cava, either directly or indirectly via the hemiazygos and accessory hemiazygos veins .
It drains the blood from the posterior walls of the thorax and abdomen. In some rare variations, the azygos vein also drains thoracic veins, bronchial veins and even gonadal veins.
It is formed by the union of the ascending lumbar veins with the right subcostal veins at the level of the 12th thoracic vertebra, ascending in the posterior mediastinum, and arching over the root of the right lung to join the superior vena cava.
Its tributaries, apart from its main tributary, the hemiazygos vein, are the bronchial veins , pericardial veins , posterior right intercostal veins . It communicates with the vertebral venous plexuses.