WEBVTT - 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. Hey, welcome

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

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

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

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

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

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

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<v Speaker 1>like three to four percent of people have a blood

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<v Speaker 1>injury phobia. But what's really interesting is that percent of

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<v Speaker 1>people faint at the sight of blood, which means there's

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<v Speaker 1>a lot of people out there who really have no

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<v Speaker 1>issue with cutting themselves, but they still faint dead away

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<v Speaker 1>any time they see themselves bleeding. That's kind of weird, right. Well,

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<v Speaker 1>when you faint from anxiety, which is what researchers think

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<v Speaker 1>is going on, when you faint from the side of

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<v Speaker 1>your own blood, your blood pressure suddenly spite, but then

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<v Speaker 1>just as quickly it decreases, and that decrease in blood

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<v Speaker 1>pressure drains blood away from your brain, causing you to

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<v Speaker 1>lose consciousness. When you're anxious or when you feel like

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<v Speaker 1>you're in danger, it's normal for your blood pressure to rise.

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

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<v Speaker 1>What's unusual in this case is the sudden decrease in

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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<v Speaker 1>hypothesis that, because your NST is also in charge of

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

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

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

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

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<v Speaker 1>of blood thing at the feet of the NST. Evolutionarily speaking,

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<v Speaker 1>passing out at the sight of your own blood doesn't

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<v Speaker 1>make much sense, and researchers have bent over backwards to

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

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<v Speaker 1>that possibly when you faint at the sight of your

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

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<v Speaker 1>bear will take you as being dead and maybe he'll

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<v Speaker 1>lose interest. Well. Another more reasonable explanation is that this

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

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

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

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

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

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<v Speaker 1>brain can be restored fairly quickly because it's a lot

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<v Speaker 1>easier for your heart to pump blood horizontally than upwards

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<v Speaker 1>against graftic check out the brain stuff channel on YouTube,

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<v Speaker 1>and for more on this and thousands of other topics,

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<v Speaker 1>visit how stuff works dot com.