Ever wondered How Do Kidneys Work? Most parents and educators know how curious kids are and how much they want to know what things are made of and how they work. Their bodies are no exception. That's why we made our app Inside the Human Body. It's also why we decided to write a series of clear and concise posts about how some of our most interesting body parts work.

To kick the series off, we're stepping inside the kidneys. We know a lot of amazing facts about them after last week's post in honour of World Kidney Day, now let's take a step closer and look at how they actually work.

What Do Kidneys Do?

Our kidneys are two bean shaped organs about the size of your fist. They're vital to maintaining general health and wellbeing. We get two because doubling up covers the possibility that one might be lost to injury or infection. Although we can get along just fine with one, with no kidney function at all, death is just a few days away.

Imagine the kidneys as a non-stop sorting and filtering system. They work sifting the recyclable from the non-recyclable waste while also cleaning your blood. Your blood supply circulates through the kidneys around twelve times an hour. They process around 200 litres of blood each day.

As part of the urinary system, your kidneys also make pee (urine) from the excess fluid and unwanted chemicals or waste in the blood. Around one to two litres of waste leaves your body as urine each day.

Kidneys also keep water levels in your body perfectly balanced. If you drink too much they make watery pee to get rid of excess. If your body is short of water, they pass less of it into urine.

How Do Kidneys Work: step 1 unfiltered blood
How Do Kidneys Work: step 2 filtered blood
How Do Kidneys Work: step 3 urine

How Do Kidneys Work?


Blood flows in and out of the kidneys through large blood vessels which divide into smaller and smaller branches upon entering the kidneys. These branches lead to a million tiny filtering units called nephrons.

Inside a nephron

Inside each nephron there's a filter called the glomerulus and a tubule. The glomerulus, which looks like a long looped tube, lets fluid and waste products pass through it. It prevents blood cells and large molecules, mostly proteins, from passing through. The filtered fluid then goes through the tubule which sends needed minerals back to the bloodstream and removes waste. What's left becomes urine. The cleaned blood returns to the body by the renal vein.

Working with the Brain

The kidneys work with the brain to control and balance water levels. When they are low, the brain releases a hormone, AHH, which makes the kidneys save water. Water levels are monitored in the brain by the Pituitary Gland.

Next time your kids ask you How Do Kidneys Work, you'll know what to answer.

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