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Aw, don't be embarrassed everyone does it! Quick Questions explains what causes blushing, which Darwin called "the most peculiar and most human of all expressions."
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Sources:
http://science.howstuffworks.com/life/blush.htm
http://www.brainpickings.org/index.php/2012/10/22/science-of-blushing/
http://guidance.nice.org.uk/IPG480
http://home.swipnet.se/sympatiska/nervous.htm
(Intro)

In every culture and every ethnicity around the world, people blush, but animals don't. And it's not just that most animals' faces are covered in fur and feathers and stuff so we can't see them blushing -- they actually don't blush.

Charles Darwin called blushing the most peculiar and most human of all expressions, which is awesome, but why do we blush?

Physiologically, we understand it. I mean, we get the mechanics of why your face turns red when Bernice sneaks up behind you and pulls your pants down in front of the whole rest of the marching band. Bernice!

Anyway, your face turns red because your sympathetic nervous system kicks in. That's the network of nerves that controls your fight or flight response. When it's triggered, it signals the release of adrenaline. Suddenly, your heart rate picks up, and you start breathing faster, and then you're ready to run away. Your pupils dilate, blood rushes to your brain so you can take in as much information as possible, and your blood vessels dilate in a process called vasodilation to improve oxygen flow.

It's basically the same effect you get from warming up before a workout. But in the very hypothetical instance of my pants being pulled down, the blood vessels in my face were responding specifically to a chemical transmitter called adenylyl cyclase. It basically tells the blood vessels in your face to let the adrenaline in.

The weird thing is that these same superficial blood vessels in your face aren't usually affected by sympathetic responses. I mean we don't blush when we're scared. If we did, that might be a sign that blushing served some sort of survival purpose. But it only happens when we're not actually in danger. So, like, why?

Well, some scientists believe that blushing evolved as a social survival trait. When you blush, then the person who's angry at you can see that you're really sorry. The person who's laughing at you can see that you're visibly embarrassed. If that sounds weird, just think about the last time your dog chewed on the furniture, then rolled onto its back when you got mad. Lots of animals have evolved this sort of behavior so they can say, "Hey, whoa, I messed up, I feel terrible."

And it's really hard to stay mad at your dog when it does that. That's how you know it's working. We may never know for sure why blushing became a thing, but it could just be our way of saying "sorry for chewing on the furniture."

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