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What do the kidneys do?

The kidneys are highly complex organs that control substances in the blood, blood pressure, and the amount of fluid in the body. They also keep the blood clean and chemically balanced.

Acting as very efficient and specialised filters, they work to rid the body of waste and toxic substances and return vitamins, amino acids, glucose, hormones and other vital substances into the bloodstream.


Where are the kidneys?

Most people have two kidneys, which sit deep in the abdomen (behind the liver and intestines) in the small of the back, either side of the spine.

Shaped like a bean, each kidney weighs anywhere between 40 grams to 190 grams depending on gender and can vary between left and right kidney. A kidney is about 10-15cms long.

Many people are able to lead healthy and active lives with just one functioning kidney. Some may have been born with one kidney, others may have had a kidney removed due to illness or injury and some people may have donated one of their kidneys to someone with kidney failure (in a kidney transplant operation).

What do kidneys do?

The kidneys have a number of important functions:

  • Their main job is to filter the blood to remove ‘waste products’ that the body doesn’t need, and to keep the amount of water in the body constant. The excess fluid and the waste products form urine.
  • They also secrete a number of essential hormones which:
    • help to regulate blood pressure by sending out signals to other parts of the body that control blood vessel contraction
    • keep our blood in a neutral non-acid state
    • stimulate red cell production from our bone marrow
    • ‘activate’ the Vitamin D we obtain from our diets and sunlight to help to keep our bones and muscles healthy.

Our kidneys must maintain these delicate balances in order to ensure that all the cells in our bodies can function properly.

Key kidney components

Around one quarter of all the blood flow leaving the heart goes to the kidneys. The blood is then channelled to about a million tiny filters in each kidney. The filters are called glomeruli. These are joined onto small tubes (tubules) to form nephrons (the structural and functional units of the kidney).

Each day around 180 litres of this filtered blood (known as filtrate) pass into the nephrons. Nearly all of the liquid is then reabsorbed back into the bloodstream: between one and one-and-a-half litres is sent out of the body as urine.

This vital process ensures that the body functions efficiently.

Sometimes kidneys do not work properly for a wide variety of reasons; some of them not yet fully understood. That’s why scientists are constantly seeking new ways to investigate and solve the many complexities of the kidney.

Make an appointment to see your GP if you think there may be something wrong with your kidney function or have any of the symptoms associated with possible kidney problems. You can find lots of helpful tips and advice about planning your visit, including what questions to ask, in our visiting your doctor section.

Links & downloads

Download our free information booklets which explains what the kidneys are and what they do.

Reviewed April 2019

The need for more research

Although the basics are known, there are still many things we don’t understand about how the kidneys develop and function normally, and how these processes can go wrong. Kidney Research UK funds research into all aspects of kidney structure, function, and disease.

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