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About the Kidneys

Before Kidney Disease can be understood it is useful to understand where the kidneys are and what they do.


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The Basics

This section describes the kidneys and how they work, and then  explores what can go wrong to cause kidney disease.

Although many people develop kidney disease for no known reason, there are certain conditions (eg, diabetes and high blood pressure) which increase the likelihood of kidney problems. It is particularly important that people with these conditions are aware of the symptoms of kidney disease, as this will help ensure early diagnosis and treatment.

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What and where are the Kidneys?

Position of the kidneys
The kidneys are bean-shaped organs about the size of a fist. They are located at the bottom of the rib cage at the back of the body.

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What do the Kidneys do?

Healthy kidneys filter the blood to:

  • Remove waste products (from metabolism of the food we eat and body cells).
  • Remove excess fluid to balance fluid levels in the body

In addition to filtering the blood, the kidneys also:

  • Help control blood pressure
  • Produce  hormones and chemicals which:
    • Help production of red blood cells
    • Maintain healthy bones

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How do the Kidneys work?

The Kidneys act like sieves, filtering the waste and excess fluid from the blood.  Blood passes through the kidneys and is cleaned before returning to the heart.

  1. First blood enters the kidneys via the renal arteries.
  2. Then, inside the kidneys, millions of mini-filtering systems called nephrons sieve the blood.
  3. Certain substances the body needs are reabsorbed and the waste products and extra fluid that the body does not need are removed in the form of urine
  4. The clean blood returns to the body through the renal veins.
  5. The urine is carried from the kidneys to the bladder by tubes called ureters.
  6. The bladder stores the urine until it is full, when the urine passes out of the body via the urethra.

Each day, the kidneysprocess about 190 litres (335 pints) of blood through 145 miles (225km) of ‘tubes’ and millions of mini filtering systems called ‘nephrons’.

In addition to filtering the blood and balancing fluid levels in the body, the kidneys also produce different hormones and chemicals, which perform several key functions.

  1. Erythropoietin is a hormone which travels in the bloodstream from the kidneys to the bone marrow where it prompts the bone marrow to make red blood cells.  Red blood cells carry the oxygen the body needs to function properly. Without healthy red blood cells, people develop anaemia, which can cause them to feel weak, cold, tired and short of breath.  Click here for more information on anaemia.
  2. Control of blood pressure – when kidneys are diseased, blood pressure increases.  High blood pressure is important - it can increase the risk of stroke, heart disease and further damage the kidneys.
  3. The kidneys maintain the correct balance between calcium and phosphate in the blood and the bones, and produce vitamin D.  So when the kidneys fail, there can be problems with the bones.

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NKF Controlled Document No. 59: About The Kidneys written: 08/06/2002 last reviewed: 16/03/2019