The kidneys clean the blood. The blood constantly flows in and out of two kidneys, leaving behind the 1% which becomes urine.
They also do some other important things. For instance, they:
1. Produce hormones that encourage the production of red blood cells
2. Control the amount of water in the blood
3. Control salt levels in the blood
4. Remove urea from the blood
Renal means relating to the kidneys.
The following story will help you remember that kidneys clean blood and control salt, water, hormones and urea.