If you want to attack this from a spiritual perspective as well as a medical one -
according to my Google search
The main purpose of the kidney is to separate urea, mineral salts, toxins, and other waste products from the blood. The kidneys also conserve water, salts, and electrolytes.

So questions I would ask myself when/if I had problems with kidneys -
what sort of poisons am I having trouble releasing in my life? How am I holding onto them, what is blocking them from being released? Am I having trouble holding onto and really absorbing or taking in things that bring me nurturance (like water?)