|Vein: Azygos vein|
|Superior vena cava, inferior vena cava, azygos vein and their tributaries. (Vena azygos labeled at center.)|
|POSTERIOR VIEW: The position and relation of the esophagus in the cervical region and in the posterior mediastinum. Seen from behind. (Azygos vein labeled at bottom left.)|
|Gray's||subject #172 667|
|Source||superior intercostal vein|
|Drains to||superior vena cava|
|Precursor||Right supracardinal vein|
The azygos vein is a vein running up the right side of the thoracic vertebral column. 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 transports deoxygenated blood from the posterior walls of the thorax and abdomen into the superior vena cava vein. The anatomy of this blood vessel can be quite variable. In some rare variations for example, it also drains thoracic veins, bronchial veins and even gonadal veins. The vein is so named because it has no symmetrically equivalent vein on the left side of the body.
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 right main bronchus posteriorly at the root of the right lung to join the superior vena cava. This "arch of the azygos vein" (arcus venae azygos) is an important anatomic landmark. As a rare anatomical variation, the arch can be displaced laterally, thereby creating a pleural septum separating an azygos lobe from the upper lobe of the right lung.
A major tributary is the hemiazygos vein, a similar structure on 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.
Azygos venous system 
The Greek root zyg refers to a pair. 'A-' means not. Thus, azygos means unpaired. The azygos vein is unpaired in that there is only one in the body, mostly on the right side. While there is the hemiazygos vein and its accessory on the left side of the body, they are considered tributaries of the azygos vein rather than its left-side equivalent.
This terminology is only accurate in some species, such as the human, dog and cat. Ruminants (such as sheep and cows) have paired azygos veins
Additional images 
See also 
- Hemiazygos vein
- Posterior intercostal veins
- Superior vena cava
- Chronic cerebrospinal venous insufficiency