V. azygos, unpaired vein, and v. hemiazygos, a semi-unpaired vein, is formed in the abdominal cavity from the ascending lumbar veins, vv. lumbales ascendentes connecting the lumbar veins in the longitudinal direction. They go up behind m. psoas major and penetrate the chest cavity between the muscle bundles of the diaphragm stem: v. azygos – together with the right n. splanchnicus, v. hemiazygos – with left n. splanchnicus or sympathetic trunk.
In the chest cavity v. azygos rises along the right lateral side of the spinal column, tightly adhering to the posterior wall of the esophagus. At level IV or V of the vertebra, it departs from the spinal column, and, bending over the root of the right lung, flows into the superior vena cava. In addition to the branches that carry blood from the organs of the mediastinum, the right lower intercostal veins fall into the unpaired vein and, through them, the veins of the vertebral plexuses. Near the place where the unpaired vein is bent over the root of the right lung, it takes in v. intercostalis superior dextra, formed from the confluence of the upper three right intercostal veins.
On the left lateral surface of the vertebral bodies behind the descending thoracic aorta lies v. hemiazygos. It rises only to the VII or VIII thoracic vertebra, then turns to the right and, passing obliquely upwards on the front surface of the spinal column behind the thoracic aorta and ductus thoracicus, flows into the v. azygos. It takes branches from the mediastinum organs and the lower left intercostal veins, as well as the veins of the vertebral plexuses. Upper left intercostal veins infuse into v. hemiazygos accessoria, which goes from top to bottom, settling down, as well as v. hemiazygos, on the left lateral surface of the vertebral bodies, and merges into either v. hemiazygos, either directly in v. azygos, leaning to the right through the front surface of the body VII of the thoracic vertebra.