Hemiazygos v.

The hemiazygos vein (vena azygos minor inferior) is a vein running superiorly in the lower thoracic region just to the left side of the vertebral column.

The hemiazygos vein and the accessory hemiazygos vein together serve as the left-sided equivalent of the azygos vein. That is, the azygos vein serves to drain most of the posterior intercostal veins on the right side of the body, and the hemiazygos vein and the accessory hemiazygos vein drain most of the posterior intercostal veins on the left side of the body. Specifically, the hemiazygos vein mirrors the bottom part of the azygos vein.

The structure of the hemiazygos vein is often variable. It usually begins as the left ascending lumbar vein or from the left renal vein, and passes upward through the left crus of the diaphragm to enter the thorax. It continues ascending on the left side of the vertebral column, and around the level of the ninth thoracic vertebra, it passes to the right across the vertebral column by passing posterior to the aorta, esophagus, and thoracic duct. On the right side, it enters the azygos vein.

The hemiazygos vein may or may not be continuous superiorly with the accessory hemiazygos vein.

It receives the 9th, 10th, and 11th posterior intercostal veins and the subcostal vein of the left side, as well as some esophageal and mediastinal veins.