Kidneys & excretion

Trillions of cells in your body are working all the time, and that produces waste substances such as urea. If these waste substances built up in the blood, that would be harmful. That is why the body has its own cleaning system: the kidneys. They constantly filter the blood and throw out what is no longer needed.

Two kidneys as filters

Most people have two kidneys. Each is about as big as a fist and shaped like a bean. They lie in the back, left and right of the spine. All of the blood flows through the kidneys again and again so it stays clean: roughly every five minutes the whole amount of blood in your body has passed through them once. Even a single kidney can do this work for the body.

How urine is formed

In the kidney the blood is pushed through fine filters. Waste substances and extra water stay behind and become urine. Important substances like sugar are taken back into the blood, they are too valuable to throw away. About 180 liters of fluid pass through these filters each day, but only about 1.5 liters leave the body as urine. So the kidney takes back almost all of the water. The finished urine collects and flows into the bladder.

The bladder as a reservoir

The urine runs from the kidneys through two thin tubes, the ureters, into the bladder. The bladder is a stretchy bag that slowly fills. At about 300 milliliters you first feel that you need the toilet, and completely full it holds around half a liter. When you pass water, the urine is then excreted from the body. This way the waste leaves the body in an orderly way.

The water balance

The kidneys also control how much water stays in the body. If you drink a lot, the kidney makes more urine, and it is pale. If you drink little or sweat heavily, the kidney saves water, and the urine gets darker. This way the amount of water in the body always stays in the right range.