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The hemiazygous vein commences at the confluence of the left subcostal vein and the left ascending lumbar vein inferior to the diaphragm. A fibrous remnant may remain between the inferior part of the hemiazygous vein and the left renal vein. This may represent the developmental left posterior cardinal vein.
At the level of T12, the hemiazygous vein ascends through the diaphragm on the left lateral surface of the vertebral bodies, posterior to the aorta and medial to the left crus. Staying posterior to the thoracic aorta and lateral to the vertebral bodies, the hemiazygous vein continues to ascend; during this course it receives the:
At the level of T8 or T9, the hemiazygous vein crosses the vertebral bodies posterior to the aorta, thoracic duct and oesophagus. On the right side of the vertebral bodies it drains into the azygous vein. Therefore, the hemiazygous vein tends to drain the right posterior thorax and lumbar regions, the lower oesophagus and possibly parts of the mediastinum.