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Azygos vein
Azygos vein is a vein in thoracic venous system that drain through side of the thoracic vertebral column. At arch of azygos, it drains itself to the superior vena cava which is the main vein in the cardiac system or the heart. It is located at the right part of thoracic wall. It mainly collects blood from posterior intercostal veins.

Structure
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. The azygos system is considered to be the azygos vein located from rib number 2 to rib number 4 while the left part of the body has the hemiazygos vein and the accessory hemiazygos vein as the venous system.

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 an anatomical variation in 1-2% of the population, 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 vertebrael venous plexuses.

The origin and anatomical course of the azygos vein are quite variable. Usually, there is a singular azygos vein on the right side of the body. However, the azygos vein is occasionally located in the midline or two independent veins may be present like in early embryonic development.

The azygos vein generally begins at the level of the lumbar vertebrae, but may originate above the point in some case. The lumbar aspect of the azygos vein can ascend anteriorly to the lumbar vertebrae and pass behind the right crus of the diaphragm or cross the aortic hiatus (where the aorta pierces the diaphragm) to the right of the cisterna chyli, a dilated lymph sac. The common trunk of the right ascending lumbar vein and the right subcostal vein join the azygos vein anterior to the body of T12. However, if the lumbar segment is absent, this trunk may form the azygos vein.

Function
The azygos vein transports deoxygenated blood from the posterior walls of the thorax and abdomen into the superior vena cava vein. The azygos system of veins is considered to be the azygos vein, along with its left-sided counterparts, the hemiazygos vein and the accessory hemiazygos vein.