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?
We want to hear all about it.
Leave it in the comments section below.
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