Azygos venous system

The azygos system of veins consists of several longitudinal vessels on both sides of the vertebral column that collect venous blood from the viscera of the mediastinum, as well as the back and the thoracic and abdominal walls. These veins ascend and empty into the superior vena cava

Some veins from the thoracic viscera may also drain into this system, and there are anastomoses with abdominal veins. These longitudinal vessels are interconnected from side to side at various points throughout their course. 

The azygos venous system is an essential anastomotic pathway that can return venous blood to the heart from the lower part of the body if the inferior vena cava is blocked.

The major veins of this system include: on the left - the azygos vein, and on the right, its two tributaries - the hemiazygos vein and the accessory azygos vein.

The origin, course, tributaries, anastomoses, and termination of these vessels are quite variable.