Why is urine yellow? – Ronan, aged 9, Greenslopes, Brisbane.
Thank you for your question, Ronan.
Our bodies use nutrients from the food we eat. But the processes involved in digestion also create what we call “byproducts”. That’s where a new chemical is created along the way.
Some of these byproducts in the body are waste and our bodies have clever waste processing systems to get rid of them.
Some of the waste goes out in your poo. And waste that can be dissolved in water goes out in your wee. We call this “water-soluble” waste. Water-soluble means it can be dissolved in water.
And the parts of your body in charge of “making” the wee are called the kidneys. They’re shaped like kidney beans.
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A delicate balance
The kidneys work around the clock to make sure the body has the right balance of water, salt and chemicals and not too much water-soluble waste in it.
Kidneys have special filters in them that help sort out the useful bits from the waste. They also are in charge of transporting the water-soluble waste from your kidneys, down two special pipes called “ureters” and into your bladder (which is down near the genitals).
When the bladder gets full, it sends a message along your nerves to your brain that makes you feel like you need to wee.
So…. why is it yellow?
One of the water-soluble waste products that your kidneys put into your urine is a chemical called urobilin, and it is yellow.
The colour of your urine depends on how much urobilin is in it and how much water is in it.
If your urine is light yellow, it means you have been drinking a lot of water and there’s a lot of water in your urine. We call this being “hydrated”.
If your urine is dark yellow, that means there’s less water, and a relatively high amount of urobilin. It probably means you haven’t been drinking enough water and could be dehydrated.
Too much water versus not enough
When you haven’t been drinking enough water, the kidneys get a message from your brain to try to keep more water in your body (and out of your bladder). You will also start to feel thirsty.
If people can’t drink water (because they have a vomiting illness, for example), they might need water put directly into their blood. This usually happens in a hospital using a drip (which is where a bag of salt water is put into your blood via a needle in your arm).
If you have been drinking more water than your body needs, the body tells the kidney filters to get rid of the spare water. That’s when your urine will look paler.
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