Medical Definition of azygos vein
: any of a system of three veins which drain the thoracic wall and much of the abdominal wall and which form a collateral circulation when either the inferior or superior vena cava is obstructed; especially : a vein that receives blood from the right half of the thoracic and abdominal walls, ascends along the right side of the spinal column, and empties into the superior vena cava—compare accessory hemiazygos vein, hemiazygos vein
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