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Hank: 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.
[Text: 'It's pretty bad, even for us.']
But 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 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. Sometimes within the first day or two, the bruise starts to turn bluish purple as white blood cells separate the hemoglobin into heme, a 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 weak, 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. Finally the bilirubin gets sent along to your liver for processing and eventually, you excrete it. But that brown hemosiderin has been sitting there the whole time, so as those other colors start to disappear, the brown becomes more visible until it gets reabsorbed as well.

So that poor guy, our friend who walked into that bar, over the next few days, his bruise probably goes from red to blue to green and finally, to yellow and brown. And after a couple of weeks, it disappears entirely. At least until the next time someone tells that sad, sad joke.

Thank you for asking and thank you to all of our supporters on Patreon who keep these answers coming with their monthly contributions. If you’d like to submit questions to be answered or get these quick questions a few days before everyone else, you can go to patreon.com/scishow. And if you have a young question to ask or in your home or in your life, you can tell them to check out our new kids channel at youtube.com/scishowkids.

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