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How do the Kidneys Work?

Mary McMahon
Updated Jan 28, 2024
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The kidneys are bean-shaped, fist-sized organs that filter the blood in the body to remove wastes and to maintain proper levels of various substances in the blood. As one might imagine, they provide a valuable function in the body, ensuring that wastes do not accumulate, potentially causing health problems.

About 20% of the blood pumped out by the heart goes directly to the kidneys via the renal artery. Once the blood enters the kidneys, it is pumped up into the nephrons, tiny structures which filter the blood. Cleaned blood is sent out from the renal vein, returning to the heart for circulation to the rest of the body, while waste products drain to the bladder through the ureters. These organs are equipped to handle a very high volume of blood every 24 hours, and each kidney can even adapt to do the job independently, in the event that one fails or is removed.

When blood enters a nephron, the nephron absorbs material that is useful to the body, allowing the rest to circulate through so that it reaches the ureter. These structures regulate the components of the blood, compensating for a large intake of salt, for example, or a reduced water intake. The goal of the kidneys is to keep the volume of water in the body constant while maintaining the composition of the blood by removing waste and keeping concentrations of various substances constant. In the process, they also keep the acid/base balance in the blood stable, regulate the body's blood pressure, maintain calcium, and stimulate the production of red blood cells.

The kidneys are quite efficient, extracting the maximum amount of value from the blood and producing surprisingly little waste. They work with the intestinal tract and the sweat glands to help remove wastes and toxins from the body, keeping the body healthy and in a state of homeostasis. However, they can break down, causing serious health problems as toxins accumulate in the blood stream, rather than being filtered out. Kidney failure can happen to the elderly and to people with renal diseases, and it requires prompt medical treatment, often including dialysis, a mechanical blood filtering treatment.

One sign that the kidneys are failing is blood in the urine, because normally they do not filter red blood cells. Doctors can also test urine to look for specific proteins and other materials which can indicate problems.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a WiseGeek researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By andee — On Nov 15, 2012

@feruze -- I have never heard that drinking too much water is hard on your kidneys, but I don't have any kind of medical background. All I have ever heard is that we don't drink enough water and still need to drink 8 glasses a day.

By SarahSon — On Nov 15, 2012

I have an uncle who is on dialysis for kidney disease. He is almost 80 years old and has been on dialysis for 2 years. For awhile he tried doing the dialysis at night in his home, but it wasn't working as well as it should. Now he has to go to the clinic 3 times a week for treatments.

He is looking into a kidney transplant, but not sure if he really wants to go through all that at his age. Even with the kidney disease, he still stays pretty active but his biggest complaint is he doesn't have the energy he had before.

By julies — On Nov 14, 2012

My sister had a bad reaction to a strong psychiatric medication and it ruined her kidneys. By the time they discovered this, her kidney function was somewhere around 25%. They immediately stopped that medication but started her on something else to help increase her kidney function.

It is really frustrating because you wonder if that will really make a difference or just make matters worse. She lives close to Mayo clinic and her doctor told her if her kidney function got any worse she would have to have a transplant.

They wouldn't even try putting her on dialysis but just go ahead and do the transplant surgery. Since she has been on this different medicine her kidney levels have only improved 2-3%, but at least they aren't worse.

By honeybees — On Nov 13, 2012

@anon285744 -- I have often heard that taking too much ibuprofen is hard on your kidneys. I know all medications have side effects, but this one is especially bad for your kidneys. Acetaminophen, which is the main ingredient in Tylenol is bad for your liver.

Every time I reach for a pain medication I wonder if it is causing damage to my kidneys or liver, but if I am in a lot of pain it is the only thing I have found that works. Whenever I have had blood work done, my kidney levels always look OK, but I try not to take ibuprofen any more than I absolutely have to.

By bear78 — On Nov 13, 2012

Is it possible to drink too much water than the kidneys can handle?

I drink a lot of water and my friend said that I shouldn't drink so much or my kidneys can't filter it all.

By discographer — On Nov 12, 2012

@ankara-- I'm very sorry to hear about your grandfather.

Urea is a toxin that is released into the bloodstream when our body metabolizes protein. Normally, the kidneys filter it out of the blood along with all other toxins and pass it out in urine. But when the kidneys are not functioning properly, urea builds up in the blood, causing damage to the body's cells.

High urea levels in blood tests is one of the first symptoms of kidney dysfunction. Doctors will usually check for this along with blood in urine if they are worried about the patient's kidneys.

Did the doctors say anything about why your grandfather's kidneys failed? Is it because of another condition, medications, etc?

By bluedolphin — On Nov 11, 2012

Thank you for this article. My grandfather died recently from kidney failure. I'm very sad but I want to learn more about the kidneys, how they work and why they sometimes fail.

The doctor told us that he died from too much urea in the blood. What does this mean?

By anon285744 — On Aug 17, 2012

Read "Medicines That Can Cause Kidney Damage" by Christine Adame The most common nephrotoxic drugs are nonsteroidal anti-inflammatory drugs (NSAIDs), some antibiotics, some painkillers and radio contrast dye used for some imaging procedures.

By laluna — On May 18, 2008

A good way to protect and care for kidneys is by eating bananas. Because bananas contain phenols they are known to fight cancer among other things. So eat bananas for you kidneys' sake.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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