Azygos v.

The azygos vein is a vein running up the right side of the thoracic vertebral column. It drains the posterior walls of the thorax and abdomen into the superior vena cava. It can also provide an alternative path for blood to the right atrium by allowing the blood to flow between the venae cavae when one vena cava is blocked.

The azygos vein is so named because it has no symmetrically equivalent vein on the left side of the body. The azygos vein can be quite variable. For example, in some cases it also drains thoracic veins, bronchial veins and even gonadal veins.

The azygos vein is formed by the union of the right ascending lumbar vein with the right subcostal vein at the level of the 12th thoracic vertebra. It ascends in the posterior mediastinum and arches over the right main bronchus posteriorly at the root of the right lung to join the superior vena cava.

The "arch of the azygos vein" (arcus venae azygos) is an important anatomic landmark. In 1-2% of the population, the arch can be displaced laterally, thereby creating a pleural septum that separates an "azygos lobe" from the rest of the upper lobe of the right lung.

A major tributary is the hemiazygos vein from the opposite side of the vertebral column. Other tributaries include the bronchial veins, pericardial veins, and posterior right intercostal veins. It communicates with the vertebral venous plexuses.