Why do I faint when I see blood?

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Fainting at the sight of blood, which is a condition called either neurocardiovascular or vasovagal syncope, is actually related in some cases to what's classified a blood injury phobia.

Something like 3-4% of people have a blood injury phobia.

But what's really interesting is that 15% of people faint at the sight of blood.

Which means there's a lot of people out there who really have no issue with cutting themselves,

but still faint dead away anyway when they see themselves bleeding.

That's kind of weird.

When you faint from anxiety, which is what researchers think is going on when you faint

from the sight of your own blood, your blood pressure suddenly spikes.

But then, just as quickly, it decreases.

And that decrease in blood pressure drains blood away from your brain, causing you to

lose consciousness.

When you're anxious or when you feel like you're in danger, it's normal for your blood

pressure to rise.

It's part of the sympathetic nervous system's "fight or flight" response.

What's unusual in this case, is the sudden decrease in blood pressure that causes you

to lose consciousness.

At the center of all this oddness is the vagus nerve.

It's a major nerve that connects your brain to various regions of your body that are involved

in involuntary movement.

Like your heart beating, your throat swallowing, that kind of stuff.

And at the other end, your vagus nerve is connected to a region of your brain called

the nucleus of the solitary tract, or the NST.

The NST is kind of like a toggle switch that goes back and forth between the sympathetic

response (your fight or flight response) or the parasympathetic response, which is what

calms you down after danger has passed.

What researchers think is going on is that the NST gets some sort of confused signal

from the vagus nerve that causes it to decrease blood pressure as part of the parasympathetic

response, without deactivating the increase in your heart rate.

Which causes a lot of blood to suddenly be pumped away from your brain, hence making

you pass out.

Another explanation is that your NST simply toggles too quickly between the sympathetic

and parasympathetic responses.

Like it's working its joystick, like, "What's going on?"

And then you're out on the floor.

Then there's another parallel hypothesis, that because your NST is also in charge of

mediating your disgust response, that there's some sort of mixture of fright and disgust

that causes you to faint, because, again, the NST is confused.

However you slice it, it seems that you can lay the fainting at the sight of blood thing

at the feet of the NST.

Evolutionarily speaking, passing out at the sight of your own blood doesn't make much

sense.

And researchers have bent over backwards to try to explain it.

What they've come up with is that, possibly, when you faint at the sight of your own blood,

say, after being mauled by a bear, the bear will take you as being dead and will lose

interest.

Pretty lame.

Another more reasonable (in my humble opinion) explanation is that this sudden decrease in

blood pressure prevents us from bleeding out of some sort of wound, and that the fainting

is just an unfortunate byproduct of the whole thing.

Either way, at any rate, whatever the case, once you're on the floor, which is usually

what happens when you faint, the blood flow to your brain can be restored fairly quickly,

because it's a lot easier for your heart to pump blood horizontally than upwards against

gravity.

Do you faint at the sight of your own blood?

We want to hear all about it.

Leave it in the comments section below.

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And while you're at it, open up a new tab and open up your world, at HowStuffWorks.com.

Hey and welcome to BrainStuff.

I'm Josh Clark, and this is the BrainStuff where I explain to you why some people faint

at the sight of their own blood.

Fainting at the sight of blood, which is a condition called either neurocardiovascular

syncope or vasovagal syncope, is actually related in some cases to what's classified

as a blood injury phobia.

Something like 3-4% of people have a blood injury phobia.

But what's really interesting is that 15% of people faint at the sight of blood.

Which means there's a lot of people out there who really have no issue with cutting themselves,

but still faint dead away anyway when they see themselves bleeding.

That's kind of weird.

When you faint from anxiety, which is what researchers think is going on when you faint

from the sight of your own blood, your blood pressure suddenly spikes.

But then, just as quickly, it decreases.

And that decrease in blood pressure drains blood away from your brain, causing you to

lose consciousness.

When you're anxious or when you feel like you're in danger, it's normal for your blood

pressure to rise.

It's part of the sympathetic nervous system's "fight or flight" response.

What's unusual in this case, is the sudden decrease in blood pressure that causes you

to lose consciousness.

At the center of all this oddness is the vagus nerve.

It's a major nerve that connects your brain to various regions of your body that are involved

in involuntary movement.

Like your heart beating, your throat swallowing, that kind of stuff.

And at the other end, your vagus nerve is connected to a region of your brain called

the nucleus of the solitary tract, or the NST.

The NST is kind of like a toggle switch that goes back and forth between the sympathetic

response (your fight or flight response) or the parasympathetic response, which is what

calms you down after danger has passed.

What researchers think is going on is that the NST gets some sort of confused signal

from the vagus nerve that causes it to decrease blood pressure as part of the parasympathetic

response, without deactivating the increase in your heart rate.

Which causes a lot of blood to suddenly be pumped away from your brain, hence making

you pass out.

Another explanation is that your NST simply toggles too quickly between the sympathetic

and parasympathetic responses.

Like it's working its joystick, like, "What's going on?"

And then you're out on the floor.

Then there's another parallel hypothesis, that because your NST is also in charge of

mediating your disgust response, that there's some sort of mixture of fright and disgust

that causes you to faint, because, again, the NST is confused.

However you slice it, it seems that you can lay the fainting at the sight of blood thing

at the feet of the NST.

Evolutionarily speaking, passing out at the sight of your own blood doesn't make much

sense.

And researchers have bent over backwards to try to explain it.

What they've come up with is that, possibly, when you faint at the sight of your own blood,

say, after being mauled by a bear, the bear will take you as being dead and will lose

interest.

Pretty lame.

Another more reasonable (in my humble opinion) explanation is that this sudden decrease in

blood pressure prevents us from bleeding out of some sort of wound, and that the fainting

is just an unfortunate byproduct of the whole thing.

Either way, at any rate, whatever the case, once you're on the floor, which is usually

what happens when you faint, the blood flow to your brain can be restored fairly quickly,

because it's a lot easier for your heart to pump blood horizontally than upwards against

gravity.

Do you faint at the sight of your own blood?