Question: If we have a really bad bruise why do they go purple.

Juan Carlos Lopez-Baez answered on 21 Mar 2014:

Hi @beccapops

Great questions! Well what happens is that some times when we bump against something, we can bleed without damaging our skin. This means that the blood vessels inside of you can become damage (they are very fragile) and blood can start coming out of it. Normally if you had broken your skin, the blood will come out and all you will need to do is press against the wound until the bleeding will eventually stop. In a bruise all that blood accumulates under your skin, forming that purple that is so characteristic of bruises. The bruise will change colour and disappear as the blood coagulates under it and it gets eaten by a special type of cell call a phagocyte (they are the big eaters of the body, they also eat the bad bacteria that attacks us).

Think about it as if you had a peach and dropped it on the floor (where you would be the peach…not literally though!). The skin of the peach will remain intact but it will become all gooey underneath. That is similar to what happens when we get a bruise and hence the expression: “you bruise like a peach” 🙂

By the way, if you want to see how a phagocyte works, check this video http://www.youtube.com/watch?v=JnlULOjUhSQ (it is a phagocyte chasing a bacteria in a petri dish)