How and why do we get bruises? – Francesca, aged 8.

Hi Francesca, thanks for sending in this great question.

If you fall off your bike or knock yourself on the coffee table at home, you might notice a blue or blackish spot on your skin a couple of days later. This is called a bruise (a contusion is the medical word for it, if you want to impress your friends).

When you bump into something, tiny blood vessels, called capillaries, can break under the skin. This causes the blood to ooze out of the vessel, kind of like a leaking water pipe. The blood that oozes out then collects under the skin. This is what gives a bruise its colour.

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Bruises can have many colours

If you get a big bruise, it will go through several colour changes before it disappears – from red, to purple, to blue, and even green and yellow.

At first, when you hurt yourself, you will have a bump on the surface of your skin. It could be a bit sore, and you might notice some redness.

The bump is because of the red cells and other fluids in the blood under the skin – that’s the stuff that oozed out of the blood vessel.

After a couple of days, the bruise will start changing into a bluish or purplish colour. The changes in colour happen because the red blood cells are breaking down. The body breaks down these red blood cells because it doesn’t need them anymore.

Finally, as the body starts to clear the unwanted red cells from under the skin, the bruise will look greenish or yellowish. It will start to fade away in about two weeks.

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What should you do if you have a bruise?

Immediately after you hurt yourself – that is, when you have a bump but it’s not blue or purple yet – you might like to apply a cold pack to the sore spot for at least five to ten minutes. A bag of frozen vegetables works well too.

If the bruise is on your arm or your leg, it can be a good idea to put it up – say on a chair or on the kitchen table.

You should tell your mum or dad or see a doctor if:

• the bruise isn’t improving or the colour is not fading away after several days
• the bruise is swollen and very painful
• you can’t move your leg or arm where the bruise has formed
• you’re getting bruises very often for no obvious reasons.

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It’s very hard to avoid getting bruises altogether. Especially for kids who like to run around, climb trees, and play sports. And the odd bruise is nothing to worry about.

But when you’re doing something like playing sport or riding a bike, as well as wearing a helmet, it can be a good idea to put on protective gear like knee guards, so you’re less likely to get hurt.

Abishek Santhakumar is affiliated with the Graham Centre for Agricultural Innovation, Charles Sturt University.