Final answer:
The renal vein carries the cleanest blood which has been filtered by the kidneys, while the renal artery brings waste-rich blood to the kidneys for filtration.
Step-by-step explanation:
The renal vein would contain the cleanest blood because it carries blood away from the kidneys after it has been filtered. The renal artery carries blood rich in waste products into the kidney. Once inside the kidney, the blood travels through arterioles and enters the nephrons where it is filtered. The clean blood then exits the kidney through the renal vein, heading towards the inferior vena cava. The renal artery and renal vein both play crucial roles in the renal circulation system, which filters the blood several times a day, maintaining the body's homeostasis.