WEBVTT - BrainStuff Classics: Why Do Some People Faint At The Sight of Blood?

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<v Speaker 1>Welcome to brain Stuff from How Stuff Works. Hi, there,

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<v Speaker 1>brain Stuff. I'm Lauren voc Obama, and I've got another

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<v Speaker 1>classic episode for you today from our former host, Christian Sager.

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<v Speaker 1>This one explores some literal brain stuff. Why do some

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<v Speaker 1>people faint at the side of blood? Hey, welcome to

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<v Speaker 1>brain Stuff. I'm Christian Sager, and this is the episode

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<v Speaker 1>where I explain to you why some people faint at

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<v Speaker 1>the sight of their own blood. Fainting at the side

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<v Speaker 1>of blood, which is either a condition called neuro cardiovascular

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<v Speaker 1>syncope or vasovagil syncope, is actually related in some cases

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<v Speaker 1>to what's classified as a blood injury phobia. Something like

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<v Speaker 1>three to four percent of people have a blood injury phobia.

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<v Speaker 1>But what's really interesting is that of people faint at

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<v Speaker 1>the sight of blood, which means there's a lot of

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<v Speaker 1>people out there who really have no issue with cutting themselves,

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<v Speaker 1>but they still faint dead away any time they see

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<v Speaker 1>themselves bleeding. That's kind of weird, right. Well, when you

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<v Speaker 1>faint from anxiety, which is what researchers think is going on,

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<v Speaker 1>when you faint from the side of your own blood,

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<v Speaker 1>your blood pressure suddenly spikes, but then just as quickly

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<v Speaker 1>it decreases, and that decrease in blood pressure drains blood

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<v Speaker 1>away from your brain, causing you to lose consciousness. When

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<v Speaker 1>you're anxious or when you feel like you're in danger,

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<v Speaker 1>it's normal for your blood pressure to rise. It's part

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<v Speaker 1>of the sympathetic nervous systems fight or flight response. What's

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<v Speaker 1>unusual in this case is the sudden decrease in blood

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<v Speaker 1>pressure that causes you to lose consciousness. At the center

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<v Speaker 1>of all this oddness is the vagus nerve. It's a

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<v Speaker 1>major nerve that connects your brain to various regions of

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<v Speaker 1>your body that are involved in involuntary movement like your

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<v Speaker 1>heart beating, your throat swallowing, that kind of stuff. And

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<v Speaker 1>at the other end, your vagus nerve is connected to

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<v Speaker 1>a region of your brain called the nucleus of the

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<v Speaker 1>solitary tract, or the NST for short. The n s

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<v Speaker 1>T is kind of like a toggle switch that goes

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<v Speaker 1>back and forth between the sympathetic response that's your fight

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<v Speaker 1>or flight response, or the parasympathetic response, which is what

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<v Speaker 1>calms you down after danger has passed. And what researchers

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<v Speaker 1>think is going on is that the NST gets some

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<v Speaker 1>sort of confused signal from the vagus nerve that causes

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<v Speaker 1>it to decrease blood pressure as part of the parasympathetic

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<v Speaker 1>response without deactivating the increase in your heart rate, which

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<v Speaker 1>causes a lot of blood to suddenly be pumped away

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<v Speaker 1>from your brain, hence making you pass out. Another explanation

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<v Speaker 1>is that your NST simply toggles too quickly between the

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<v Speaker 1>sympathetic and parasympathetic responses, like it's working like a joystick

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<v Speaker 1>and it's going back up and down, and you your

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<v Speaker 1>brain is saying what's going on? And then while you

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<v Speaker 1>pass out on the floor. Then there's another parallel hypothesis

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<v Speaker 1>that because you're n s T is also in charge

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<v Speaker 1>of mediating your disgust response, that there's some sort of

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<v Speaker 1>mixture of fright and disgust that causes you to faint,

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<v Speaker 1>because again the NST is confused. However you slice it,

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<v Speaker 1>it seems that you can lay the fainting at the

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<v Speaker 1>sight of blood thing at the feet of the NST.

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<v Speaker 1>Evolutionarily speaking, passing out at the sight of your own

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<v Speaker 1>blood doesn't make much sense, and researchers have bent over

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<v Speaker 1>backwards to try to explain it. What they've come up

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<v Speaker 1>with is that possibly when you faint at the sight

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<v Speaker 1>of your own blood, say, after being mauled by a bear,

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<v Speaker 1>the bear will take you as being dead and maybe

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<v Speaker 1>he'll lose interest. Well. Another more reasonable explanation is that

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<v Speaker 1>this sudden decrease in blood pressure prevents us from bleeding

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<v Speaker 1>out of some sort of wound, and the fainting is

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<v Speaker 1>just an unfortunate byproduct of the whole thing. Either way,

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<v Speaker 1>whatever the case, once you're on the floor, which is

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<v Speaker 1>usually what happens when you faint, the blood flow to

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<v Speaker 1>your brain can be restored fair really quickly, because it's

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<v Speaker 1>a lot easier for your heart to pump blood horizontally

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<v Speaker 1>than upwards against grab. Today's episode was produced by Tyler

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<v Speaker 1>Clang and written by myself with style notes from Josh

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<v Speaker 1>Clark for brain Stuff's Erstwhile YouTube series. If you like

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<v Speaker 1>our show and also like not being naked, check out

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<v Speaker 1>our March shop at t public dot com, slash brain stuff,

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<v Speaker 1>and of course, for more on this and lots of

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<v Speaker 1>other topics that attempt to explain the unexplained, visit our

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<v Speaker 1>home planet how stuff Works dot com.