Kidney.
To answer your question:
Renal physiology.
For optimum kidney health, drink plenty of water, get daily exercise, including 30-min cardio sessions three times a week, and well-rounded (all muscles) anerobic (weights) exercises twice a week, and eat a healthy, well-balanced diet of fresh fruits and vegetables, some lean meats, dairy, grains, cereals, nuts, and legumes. Avoid processed food, high-fructose corn syrup, artificial sweeteners, and alcohol, other than in moderation.
Of course this applies to optimum health for your entire body, too!
