Wikijunior:Human Body/Digestive System/Kidneys
Kidneys "are bean shaped organs that serve several essential regulatory roles in vertebrate animals. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintaining salt and water balance). They serve the body as a natural filter of the blood, and remove wastes, which are diverted to the urinary bladder. In producing urine, the kidneys excrete wastes such as urea and ammonium, and they are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol, erythropoietin, and the enzyme renin."
What do they do?
Kidneys separates the waste from the nutrients using nephrons. Then, the nutrients are taken up to the body where it will be useful and help us with our other systems. Next, the extra water or wastes go down through a tube called the uterus, which then goes to your bladder.
When you go to the bathroom, you push the urine out of your bladder and it goes to the penis, which then pushes down the urine. Now, your healthy and the wastes are gone.
Are they useful?
How can we protect our kidneys?
Eat the right foods and exercise daily. Also, when you have to go and use the bathroom, its a good idea to go. Or else, you will form "solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine."  called Kidney stones. They form in the kidneys and hurts your body badly. Don't hold your urine inside your bladder, or you will get kidney stones.
Drink a lot of water also, and exercise a lot!
Well... More will come later! :-)