The kidneys are a pair of bean-shaped organs on each side of your spine, under your ribs and behind your belly. Each kidney is about 4 or 5 inches long, roughly the size of a large fist. It goes to the kidneys to extract water from the excretory system and put it back into the blood. Kidneys filter your blood. They eliminate wastes, control the body's fluid balance, and hang onto the right levels of electrolytes. All of the blood in your body passes through them quite a few times a day.