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How do the kidneys work?

Last Update: January 7, 2015; Next update: 2018.

A healthy person has two kidneys, which look like oversized beans. The kidneys are located to the right and left of the spine, just below your ribcage.

Each kidney is about as big as a bar of soap or the size of your fist. It weighs between 135 and 150 grams. The indented side of the kidney faces inward toward the spine. A tube called the ureter leaves the kidney at the middle of this indented side. Nerves, and blood and lymph vessels also lead in and out of the kidney here. Each kidney is covered by three layers made out of fat or connective tissue, called the kidney capsule (or renal capsule). These layers give the kidney extra stability, protect it from external harm and attach it to the surrounding tissue.

The kidney is surrounded by a thin outer renal cortex. There are about 2.4 million renal corpuscles in the renal cortex. Urine is made here. The renal medulla is found inside the kidney. Blood vessels and winding renal tubules run through the renal medulla. Urine passes through these renal tubules, then through the renal pelvis to the ureter, and then on to the urinary bladder.


Illustration: Kidney section (side view)

The kidneys are our body’s sewage treatment plants: They produce urine, getting rid of waste products that form in the body or that we have consumed in food and drinks. Examples of waste products include ammonia and urea, which are made when proteins are broken down. The body is also able to get rid of medicine, drugs or toxins though urine.

Besides producing urine, the kidneys also have other important tasks. They regulate the body’s water balance by either holding back water or releasing it with the urine. When the kidneys hold back water, more fluid enters the blood vessels. The volume of blood increases, and the blood pressure rises. When the kidneys release more water, the volume of blood decreases and the blood pressure falls. There are also special cells in the kidneys that produce the protein renin. This enzyme causes blood pressure to rise as well.

The kidneys keep the acidity (the pH value) of the blood as constant as possible too, to make sure that the blood does not get too acidic or too alkaline. They also produce two important hormones: calcitriol and erythropoietin. Calcitriol is the active form of vitamin D and regulates things like the amount of calcium in the body. Erythropoietin plays a role in the production of red blood cells. If there is too little sugar (glucose) in the blood, the kidney can make sugar from the amino acid glutamine and release it into the bloodstream.

Each kidney has a pyramid-shaped adrenal gland “sitting” on it – like a small hat. But the adrenal glands have very different functions to the kidneys. They are glands that produce a lot of important hormones.


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  • Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.
  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.
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    Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. We do not offer individual consultations.

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