Kidney

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"). The average person has 1 to 1½ gallons of blood circulating through his or her body. The kidneys filter that blood as many as 400 times a day! More than 1 million tiny filters inside the kidneys remove the waste. These filters, called nephrons are so small you can see them only with a high-powered microscope.

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 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 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

The kidneys also balance the volume of fluids and minerals in the body. This balance in the body is called homeostasis. If you put all of the water that you take in on one side of a scale and all of the water your body gets rid of on the other side of a scale, the sides of the scale would balance. Your body gets water when you drink it or other liquids. You also get water from some foods, like fruits and vegetables.

Water leaves your body in several ways. It comes out of your skin when you sweat, out of your mouth when you breathe, and out of your urethra in urine when you go to the bathroom. There is also water in your bowel movements (poop).

When you feel thirsty, your brain is telling you to get more fluids to keep your body as balanced as possible. If you don't have enough fluids in your body, the brain communicates with the kidneys by sending out a hormone - ADH - anti diuratic hormone - that tells the kidneys to hold on to some fluids. When you drink more, this hormone level goes down, and the kidneys will let go of more fluids and these fluids will be used in urea and passed out of the body as urine.