The word ‘azygos’ means that which is unpaired. The word is derived from the word zygote, which means paired. The azygos vein is called so because it is the only vein in the body, which is not in pair.

The azygos vein begins opposite the first or second lumbar vertebra, by a branch, the ascending lumbar vein; sometimes by a branch from the right Renal vein, or from the Inferior Vena Cava. It enters the Thorax through a opening called the aortic hiatus in the Diaphragm, and passes along the right side of the Vertebral Column to the fourth thoracic vertebra, where it arches forward over the root of the right lung, and ends in the Superior Vena Cava, just before that vessel enters the Heart. In the aortic hiatus, it lies with the Thoracic Duct on the right side of the Aorta; in the Thorax it lies upon the intercostal Arteries, on the right side of the Aorta and Thoracic Duct, and is partly covered by Pleura (covering of the Lungs).

HEMIZYGOS VEIN: The azygos vein on the left side is called the Hemiazygos vein. The hemiazygos vein arises from one or more of three possible roots (lateral, intermediate and medial). The lateral root arises in 85% of individuals from the left subcostal and left ascending lumbar vein at the head of the twelfth rib. The intermediate root occurs in only 28% of individuals and arises from the dorsal side of the left Renal vein. The medial root has an incidence of 40% and arises in a plexiform manner from small Veins on the anterior side of upper lumbar Veins and communicates with the Inferior Vena Cava, Renal vein, and possibly from the second and third lumbar segmental. The hemiazygos vein usually receives the ninth to eleventh posterior intercostal Veins, and may include the eighth. The hemiazygos vein passes behind the Aorta from the left side to join the right-sided azygos. The hemiazygos Veins have been found passing to the right side of the body ventral to (cross over) the Aorta. In situs inversus and persistent left Superior Vena Cava, the hemiazygos may switch sides with the azygos vein. In 40% of cases the hemiazygos is continuous above with an accessory hemiazygos vein. The vein may open into the Right Atrium, brachiocephalic trunk, internal thoracic vein, left Subclavian vein (ascribed to an elongated embryonic left common cardinal vein), or left brachiocephalic (after receiving abnormally the azygos vein). Tributaries: Its main tributary is the hemiazygos vein. Other tributaries are the bronchial Veins, pericardial Veins, and posterior right intercostal Veins. It communicates with the vertebral venous plexuses.

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.

Primary anomalies of the azygos vein generally result from intrathoracic tumor compression or Inferior Vena Cava interruption with azygos vein continuation. Under developed Inferior Vena Cava with Azygos continuation leads to recurrent Deep Vein Thrombosis in the leg.