Kidneys are paired organs that purify the blood, controls body fluid and electrolytes and also process the waste out of the body. It is bean-shaped, of about 4 to 5 inches in length that roughly resembles the size of a fist. It sits beneath the ribs, just behind the belly. It purifies the blood by filtering them, from where it absorbs back the minerals, salts, and water, and removes the waste in the form of urine.