Everybody knows that some organs in the human body are necessary for survival: you need your brain, your heart, your lungs, your kidneys . . .

KIDNEYS? Absolutely. Even though you won't find a Valentine's Day card with a kidney on the cover, the kidneys are every bit as important as the heart. You need at least one kidney to live!

Kidneys normally come in pairs. If you've ever seen a kidney bean, then you have a pretty good idea what the kidneys look like. Each kidney is about 5 inches (about 13 centimeters) long and about 3 inches (about 8 centimeters) wide - about the size of a computer mouse.

To locate your kidneys, put your hands on your hips, then slide your hands up until you can feel your ribs. Now if you put your thumbs on your back, you will know where your kidneys are. You can't feel them, but they are there. Read on to find out more about the cool kidneys.

Cleaning Up
One of the main jobs of the kidneys is to filter the waste out of the blood. How does the waste get in your blood? Well, your blood delivers nutrients to your body. Chemical reactions occur in the cells of your body to break down the nutrients. Some of the waste is the result of these chemical reactions. Some is just stuff your body doesn't need because it already has enough. The waste has to go somewhere; this is where the kidneys come in.

First, blood is carried into the kidneys by the renal artery (anything in the body related to the kidneys is called renal). About 400 gallons (1,514 liters) of recycled blood are pumped through the kidneys every day! Then the waste is collected out of the blood by tiny filters inside the kidneys. These filters are called nephrons, and they are so tiny they can only be seen with a high-powered microscope! There are over a million nephrons inside each kidney.

The waste that is collected combines with water (which is also filtered out of the kidneys) to make urine (pee). As each kidney makes urine, the urine slides down a long tube called the ureter (say: yur-uh-ter) and collects in the bladder, a storage sac that holds the urine. When the bladder is about halfway full, your body tells you to go to the bathroom. When you pee, the urine goes from the bladder down another tube called the urethra (say: yu-ree-thruh) and out of your body.

The kidneys, the bladder, and their tubes are called the urinary system. Here's a list of all of the parts of the urinary system:

• the kidneys: filters that take the waste out of the blood and make urine
• the ureters: tubes that carry the urine to the bladder
• the bladder: a bag that collects the urine
• the urethra: a tube that carries the urine out of the body

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