December 28, 2016 • 4 mins
Christian explains the science of why people faint at the sight of blood.
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Episode Transcript
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Speaker 1 (00:02):
Welcome to Brain Stuff from How Stuff Works. Hey, welcome
to Brain Stuff. I'm Christian Sager, and this is the
episode where I explain to you why some people faint
at the sight of their own blood. Fainting at the
side of blood, which is either a condition called neuro
cardiovascular syncope or vasovagil syncope, is actually related in some
(00:27):
cases to what's classified as a blood injury phobia. Something
like three to four percent of people have a blood
injury phobia. But what's really interesting is that percent 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 they still faint dead away
any time they see themselves bleeding. That's kind of weird, right. Well,
(00:52):
when you faint from anxiety, which is what researchers think
is going on, when you faint from the side of
your own blood, your blood pressure suddenly spite, 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.
(01:14):
It's part of the sympathetic nervous systems 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
(01:37):
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 for short. The n
s T is kind of like a toggle switch that
goes back and forth between the sympathetic response that's your
fight or flight response, or the parasympathetic response, which is
(02:00):
what calms you down after danger has passed. And 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
(02:21):
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 like a
joystick and it's going back up and down, and you
your brain is saying what's going on? And then while
you pass out on the floor. Then there's another parallel
(02:43):
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,
(03:05):
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 maybe he'll
lose interest. Well. Another more reasonable explanation is that this
(03:27):
sudden decrease in blood pressure prevents us from bleeding out
of some sort of wound, and the fainting is just
an unfortunate byproduct of the whole thing. Either way, 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
(03:47):
easier for your heart to pump blood horizontally than upwards
against graftic check out the brain stuff channel on YouTube,
and for more on this and thousands of other topics,
visit how stuff works dot com.
Welcome to Brain Stuff from How Stuff Works. Hey, welcome
to Brain Stuff. I'm Christian Sager, and this is the
episode where I explain to you why some people faint
at the sight of their own blood. Fainting at the
side of blood, which is either a condition called neuro
cardiovascular syncope or vasovagil syncope, is actually related in some
(00:27):
cases to what's classified as a blood injury phobia. Something
like three to four percent of people have a blood
injury phobia. But what's really interesting is that percent 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 they still faint dead away
any time they see themselves bleeding. That's kind of weird, right. Well,
(00:52):
when you faint from anxiety, which is what researchers think
is going on, when you faint from the side of
your own blood, your blood pressure suddenly spite, 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.
(01:14):
It's part of the sympathetic nervous systems 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
(01:37):
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 for short. The n
s T is kind of like a toggle switch that
goes back and forth between the sympathetic response that's your
fight or flight response, or the parasympathetic response, which is
(02:00):
what calms you down after danger has passed. And 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
(02:21):
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 like a
joystick and it's going back up and down, and you
your brain is saying what's going on? And then while
you pass out on the floor. Then there's another parallel
(02:43):
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,
(03:05):
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 maybe he'll
lose interest. Well. Another more reasonable explanation is that this
(03:27):
sudden decrease in blood pressure prevents us from bleeding out
of some sort of wound, and the fainting is just
an unfortunate byproduct of the whole thing. Either way, 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
(03:47):
easier for your heart to pump blood horizontally than upwards
against graftic check out the brain stuff channel on YouTube,
and for more on this and thousands of other topics,
visit how stuff works dot com.
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Josh Clark
Jonathan Strickland
Ben Bowlin
Lauren Vogelbaum
Cristen Conger
Christian Sager
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