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Why Do Bruises Change Colors?

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(Intro music)

Alright, you ready for a joke?

A man walks into a bar.

Ouch.

Normally the joke ends there, if you could call that a joke.

We here at SciShow take our puns very seriously, and can't help but wonder what happens to that guy after he whacks his head into the bar.

Well, he probably ends up with a bruise which goes through a rainbow of colors over the next several days.

Using the power of science we could make some educated guesses about how exactly those colors will progress, and why.

When you get a bruise it's because you banged yourself hard enough to break capillaries

Those tiny little blood vessels that are everywhere inside your body.

After they break, the blood leaks outside of the capillaries which is why bruises tend to be reddish at first.

It's because of red iron, carrying hemoglobin, which is what your blood uses to cart oxygen around your body.

All those other colors like, blue and green and yellow and brown show up as the body breaks down the hemoglobin and reabsorbs its component parts.

Sometime, within the first day or two, the bruise starts to turn bluish-purple as white blood cells separate the hemoglobin

into heme, the compound that contains the iron, and globin, a globular protein.

The globin gets broken down into amino acids and carried away by the white blood cells.

Meanwhile, the iron from the heme becomes part of a compound called hemosiderin

which gives bruises their brownish color, but it usually doesn't become visible until later.

After about a week the rest of the heme molecule gets broken down by an enzyme

into another compound that happens to be green called biliverdin.

Then the biliverdin gets broken down by another enzyme to form Bilirubin which turns the bruise yellow.