Answer Dr Franz Schelling: "Venous blood leaves the spinal cord via tiny veins, running along nerve roots, which empty into a dense mesh of closely interconnected veins that encircle the spinal dural sac in the spinal canal. Through the veins of the intervertebral foramina this large venous mesh connects again to two venous channels running along the right respectively left side of the vertebral bodies of lumbar and thoracic spine. Within the chest, the left channel, here referred to as hemiazygos vein, usually drains its blood through one or two venous channels turning anteriorly around the vertebral column into its right-sided counterpart, the azygos vein. This azygos vein finally attains, in bending anteriorly over the right lung, the superior cava vein. In some people the left-sided hemiazygos vein drains its blood upwards into the left brachiocephalic vein.  

Inside the abdomen, the right and left-sided venous channel attending the lumbar spine downwards in direction of internal iliac vein are both named ascending lumbar vein. The right ascending lumbar vein has a series of short branches which open directls frontwards into inferior cava vein. The left ascending lumbar vein compensates for these connections to inferior cava vein in gaining a wide access to left renal vein. Crossing over to the right to inferior cava vein, the left renal vein passes between lumbar spine plus abdominal aorta behind and mesenteric artery, pancreas, stomach plus abdominal wall in front. The great quantities of blood coming from left kidney easily overburden the entire spinal venous drainage once the left renal vein comes to be obstructed or compressed.

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