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17 September 2014
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You are here: BBC Science > Human Body & Mind > The Body > Organs
Fact files

Organs - Kidneys

System: Urinary

Location: At the bottom of your ribcage and towards the back of your body

Physical description: Fist sized, dark red and kidney bean-shaped

Function: To make urine from waste products and excess water found in your blood

Balancing your blood

For your body to work properly, the conditions inside it, such as water, pH and salt levels, need to be kept constant. Your kidneys play a vital role in keeping your blood composition constant. They filter your blood to remove excess water and waste products, which are secreted from your kidneys as urine.

One quarter of your blood supply passes through your kidneys every minute. It enters your kidney and is distributed to minute filtration units known as nephrons. Each of your kidneys contains more than one million nephrons. The main substances your nephrons filter out of your blood are:

  • Water
  • Nitrogen-containing compounds like urea that are produced when your body breaks down proteins
  • Salts
  • Acids
  • Alkalis

Your nephrons filter these substances out of your blood and then reabsorb some of them back into your blood. This keeps your blood composition constant. Excess water and waste products are then secreted as urine. Your kidneys vary the amount of a substance that is reabsorbed into the blood or secreted as urine. This determines the volume and composition of your urine. For example, when you drink a lot of water, your kidneys produce a lot of urine to stop the water levels in your body getting too high. But, if you don't drink much, your kidneys only produce a small amount of concentrated urine, keeping as much water as possible in your body.

In 24 hours, your kidneys filter around 150 litres of blood and produce roughly 1.5 litres of urine.

Regulating blood pressure

When your kidneys detect that your blood pressure is dropping, they secrete an enzyme called renin. This enzyme triggers a chain of events that makes your kidneys reabsorb more salt and water, leading to an increase in blood pressure.

When kidneys go wrong

People can live healthily with one functioning kidney. However, when about 90% of kidney function has been lost, a person can only survive by having dialysis. Dialysis works by using a machine that replicates the blood-cleaning function of healthy kidneys. In the most extreme cases of kidney failure, survival depends on the person receiving a donor organ.

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