The ascending lumbar vein is a paired structure (i.e. one each for the right and left sides of the body). It starts at the lateral sacral veins, and it runs superiorly, intersecting with the lumbar veins as it crosses them.
When the ascending lumbar vein crosses the subcostal vein, it becomes one of the following:
the azygos vein (in the case of the right ascending lumbar vein)