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Other Terms: Azygos venous tree, Vena azygos, Veine azygos
The azygos vein begins in the abdomen and is the continuation of the right ascending lumbar vein. Entering the thorax through the aortic opening in the diaphragm, it runs upward in the posterior mediastinum to the right side of the bodies of the thoracic vertebrae to the fourth thoracic vertebra. Here, it arches anteriorly over the right bronchus to empty into the superior vena cava. In the abdomen, it lies behind the right crus of the diaphragm upon the first or second lumbar vertebra, and to the right of the thoracic duct and the abdominal aorta. It may communicate with the renal vein, or the inferior vena cava. By means of the ascending lumbar vein, it connects the iliac veins with the superior vena cava. In the posterior mediastinum it lies to the right of the esophagus and aorta, and anterior to the lower right intercostal arteries and the thoracic vertebrae. On its right side are the pleura and lung, and in front of it are the root of the right lung and the pleura. Its valves are imperfect. It receives the hemiazygos vein; the accessory hemiazygos, or some of the left intercostal veins; the lower end of the lower left superior intercostal vein occasionally; the lower esophageal veins of the right side; the right bronchial vein; the lower right superior intercostal vein; the mediastinal veins; the nine lower intercostal veins of the right side, and the right subcostal vein.