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  • Written by Brooke Huuskes, Lecturer in Human Anatomy, Physiology Anatomy & Microbiology, La Trobe University

Curious Kids is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

Why do we have two kidneys when we can live with only one? – Question from the students of Ms Morris’ Grade 5 class, Ringwood North Primary School, Victoria.

This is a really great question. The answer is scientists are not completely sure but we do have some theories. That is often the case with science.

Most of the animals you see above ground on Earth today, including humans, are the same on both sides. We have two eyes, two ears, and even two nostrils. Scientists gave this a fancy name called “bilateral symmetry.”

If you look in the mirror and draw an imaginary line down the middle of your reflection you will see that you have an arm and a leg on each side. If you had goggles that let you see your insides, you would see that you also have a kidney and a lung on each side too.

But it wasn’t always like this. And some animals still only have one kidney.

Around 500 million years ago, our long-lost relatives that were living in the ocean (some of whom probably only had one kidney) decided to leave the water to walk and live on land.

This was a very important moment in our history because on land, animals could change to grow a very complicated body with all of the important organs that are inside you, including two kidneys.

Read more: Curious Kids: how does my tummy turn food into poo?

Two kidneys better than one?

Right now, your kidneys are getting rid of all things your body does not need. They do this by “cleaning” your blood.

All of this waste will exit your body when you go to the toilet to pee. But your kidneys do a lot more than just clean your blood. They help your bones stay healthy, tell your body when to make new blood cells, and even help you stay upright when you’re walking around all day by taking care of your blood pressure.

With all those important functions, scientist think having two kidneys must be important for our survival.

Kidneys ‘clean’ your blood and send waste to your bladder, so you can pee it out. Shutterstock

Growing up with one kidney

It is true, you can live with only one kidney. Some people are born with only one because the other one did not grow properly. Other times, the two kidneys touch each other when they are first growing and join together, making one kidney shaped like a horseshoe. People with these types of kidneys have to be very careful because they might get sick more easily than someone who has two kidneys.

Needing an extra kidney

Sometimes our kidneys stop working. When this happens our blood cannot be cleaned and we can get very sick. The only way to stay alive is to be attached to a big machine that cleans your blood for you, or have a kidney transplant.

This happened to me when my kidneys stopped working properly. My dad gave me one of his kidneys. Thanks, Dad.

There are two people involved in a kidney transplant: a donor who is going to give their kidney, and a recipient who will receive the kidney.

After the new kidney is put into the recipient, both the donor and recipient only have one kidney that works properly. Both the donor and the recipient can live long happy lives with only one kidney. They just have to take extra care that they eat healthily and exercise to stay fit. One person living in Australia has been using a transplanted kidney for 45 years!

So, while your body works best when all of your organs are inside you and working properly, scientists still don’t exactly know why we have two kidneys. However, it is good to know that we have a few spare parts that we can live without.

And if you’re an adult reading this, it’s good to make sure you are registered as an organ donor and also chat to your family so they know you want to donate. You may one day save a life.

Read more: Curious Kids: why are burps so loud?

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