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The renal vein carries blood AWAY from the kidneys after urea has been filtered out and excreted in urine, so it has the lowest urea concentration. The renal artery brings blood TO the kidneys with high urea content that needs to be removed. The dorsal aorta and posterior vena cava carry systemic blood with intermediate urea levels. Since the kidneys' job is to remove urea from blood, the blood leaving them (renal vein) has the least amount of it.