How Your Kidneys Work

What happens

• Your two kidneys sit below your ribs at the back. Each has about a million tiny filters called nephrons.
• Every day they filter about 180 litres of fluid from your blood and take almost all of it back. Only 1–2 litres leave as urine.
• They also balance water and salt, help control blood pressure, and help make red blood cells, strong bones and the right acid level.
• About 1 in 7 US adults has chronic kidney disease, and about 9 in 10 of them don't know it. Early kidney disease usually has no symptoms.
• Diabetes and high blood pressure are the most common causes. A urine test and a blood test can find kidney disease early.

Protecting your kidneys

• Keep your blood pressure and blood sugar in the range your care team recommends.
• Ask about a urine albumin test and a blood test (eGFR) if you have diabetes, high blood pressure, heart disease, or kidney failure in your family.
• Be careful with anti-inflammatory pain relievers like ibuprofen and naproxen. If you need them regularly, ask your pharmacist or doctor.
• Don't smoke, stay active and cut back on salt.
• Drink regularly through the day, more when it's hot or you're active. If you have kidney or heart failure, follow your care team's advice on fluids.

Common questions

What are the early signs of kidney disease?

Early kidney disease usually has no symptoms, so many people feel fine until their kidneys are badly damaged. That's why testing matters: a urine test for albumin and a blood test that estimates your GFR can find it early.

Do I need to drink eight glasses of water a day for my kidneys?

There's no single number that fits everyone. Your needs depend on your body, the weather and how active you are, and water from food and other drinks counts too. Drink regularly, a bit more when it's hot or you're active, and aim for pale yellow urine. If you have kidney or heart failure, follow your care team's advice on fluids.

Can ibuprofen hurt your kidneys?

Anti-inflammatory pain relievers such as ibuprofen and naproxen can harm the kidneys if they are used often or taken when you're dehydrated. If you need pain relief regularly, or you already have kidney disease, ask your pharmacist or doctor which option is safest for you.

Transcript

Place your hands on your back, just below your ribs, on either side of your spine. Tucked in there are two of the hardest-working organs you have: your kidneys.

They're small, but every minute of every day, they clean your blood and keep your body in balance. Let's see how your kidneys work, and how to look after them.

Each kidney is shaped like a bean, and is about the size of your fist.

And they get a big share of your blood. About a fifth of the blood pumped by your heart flows to your kidneys, through the renal arteries.

Cleaned blood leaves through the renal veins. The waste drains away as urine, down two thin tubes called ureters, into your bladder.

Inside each kidney are about a million tiny filtering units, called nephrons.

Each nephron starts with a glomerulus, a tiny ball of blood vessels that works like a sieve. Water, salts, sugar and wastes pass through. Blood cells and most proteins stay in the blood.

The filtered fluid then flows along a long, winding tube, called the tubule.

Here's the remarkable part. Every day, your kidneys filter about a hundred and eighty litres of fluid out of your blood. That would fill about ninety bottles, each holding two litres.

But almost all of it is taken back. The tubule returns the water, salts and sugar your body needs to the blood. Only about one to two litres a day is left over, as urine.

Your kidneys do much more than filter. They balance the water and salts in your body, and they help control your blood pressure.

They make a hormone that tells your bone marrow to build red blood cells. They turn vitamin D into its active form, to keep bones strong. And they remove acid, to keep your blood in balance.

When the kidneys are damaged and slowly lose their ability to filter over months or years, that's called chronic kidney disease.

Kidney disease is common. In the United States, about one in seven adults has chronic kidney disease, and about nine in ten of them don't know it.

That's because early kidney disease usually has no symptoms. Many people feel fine until their kidneys are badly damaged.

The two most common causes are diabetes and high blood pressure. Over the years, high blood sugar and high pressure damage the tiny blood vessels in the kidney's filters.

That's why testing matters. If you have diabetes, high blood pressure, heart disease, or kidney failure in your family, ask about two simple tests.

A urine test looks for albumin, a protein that leaks through damaged filters. And a blood test measures a waste called creatinine, to estimate how well your kidneys are filtering. That's your GFR.

Finding it early matters, because treatment can slow the damage. If the kidneys ever fail, dialysis or a kidney transplant can take over their work.

The good news is that there's a lot you can do to protect your kidneys. Start by keeping your blood pressure and blood sugar in the range your care team recommends.

Be careful with anti-inflammatory pain relievers, like ibuprofen and naproxen. Used often, or when you're dehydrated, they can harm the kidneys. If you need them regularly, ask your pharmacist or doctor.

Don't smoke. Stay active, and cut back on salt. Each of these helps keep your blood pressure and blood vessels healthy, including the ones in your kidneys.

One common myth is that everyone needs eight glasses of water a day to flush out their kidneys. In fact, there's no magic number.

Your needs depend on your body, the weather and how active you are, and water from food and other drinks counts too. Drink regularly through the day, a bit more when it's hot or you're active, and aim for pale yellow urine. If you have kidney or heart failure, follow your care team's advice on fluids.

So, to recap. Your kidneys filter your blood. They take back what you need. They keep your body in balance. And simple tests can catch problems early.

Now you know how your kidneys work, and how to look after them. Two quiet filters, working for you day and night.

Sources

1. NIDDK: Your Kidneys & How They Work
2. CDC: Chronic Kidney Disease in the United States
3. NIDDK: Chronic Kidney Disease Tests & Diagnosis
4. NIDDK: Causes of Chronic Kidney Disease
5. NIDDK: Managing Chronic Kidney Disease
6. NIDDK: Keeping Kidneys Safe — Smart Choices About Medicines
7. Valtin H. "Drink at least eight glasses of water a day." Really? Am J Physiol 2002