Kidneys

The kidneys are a pair of organs that clean your blood. They sit in the back of your belly, one on each side of your spine, just below your ribs. Each kidney is shaped like a large bean and is about the size of your fist. Together, the two kidneys do one of the most important jobs in the body. They take out waste and extra water and turn it into urine.

Blood flows through your kidneys all day and all night. About every minute, half a cup of blood enters each kidney. The kidneys pull out things the body does not need, like leftover salts and a waste product called urea. They also save things the body does need, like water and sugar, and send them back into the blood. The waste leaves the kidneys as urine, which travels down two tubes called ureters and is stored in the bladder.

Inside each kidney are about a million tiny filters called nephrons. A nephron is so small you need a microscope to see it. If you stretched out all the nephrons from one kidney into a single line, the line would be more than 50 miles long. That is longer than two marathons.

Kidneys do more than make urine. They also help keep your blood pressure steady, tell your bones to stay strong, and signal the body to make more red blood cells when you need them. They check the amount of water in your blood every few seconds. If you drink a big glass of water, your kidneys notice within minutes and start making more urine. If you forget to drink water on a hot day, they hold water back so you do not dry out.

Most people are born with two kidneys, but you can live a healthy life with just one. That is why a person can give a kidney to someone who needs one. This is called a kidney transplant, and doctors perform thousands of them each year. Sometimes a sick kidney cannot filter blood anymore. People with that problem use a machine called a dialysis machine, which does the kidneys' job for them.

You can help your kidneys stay healthy. Drinking water is the simplest thing. Eating fruits and vegetables, moving your body, and not eating too much salt all matter too. Your kidneys are quiet workers, but they keep your whole body in balance.