Speaker 1: Welcome to brain Stuff from How Stuff Works. Hi, there,
Speaker 1: brain Stuff. I'm Lauren voc Obama, and I've got another
Speaker 1: classic episode for you today from our former host, Christian Sager.
Speaker 1: This one explores some literal brain stuff. Why do some
Speaker 1: people faint at the side of blood? Hey, welcome to
Speaker 1: brain Stuff. I'm Christian Sager, and this is the episode
Speaker 1: where I explain to you why some people faint at
Speaker 1: the sight of their own blood. Fainting at the side
Speaker 1: of blood, which is either a condition called neuro cardiovascular
Speaker 1: syncope or vasovagil syncope, is actually related in some cases
Speaker 1: to what's classified as a blood injury phobia. Something like
Speaker 1: three to four percent of people have a blood injury phobia.
Speaker 1: But what's really interesting is that of people faint at
Speaker 1: the sight of blood, which means there's a lot of
Speaker 1: people out there who really have no issue with cutting themselves,
Speaker 1: but they still faint dead away any time they see
Speaker 1: themselves bleeding. That's kind of weird, right. Well, when you
Speaker 1: faint from anxiety, which is what researchers think is going on,
Speaker 1: when you faint from the side of your own blood,
Speaker 1: your blood pressure suddenly spikes, but then just as quickly
Speaker 1: it decreases, and that decrease in blood pressure drains blood
Speaker 1: away from your brain, causing you to lose consciousness. When
Speaker 1: you're anxious or when you feel like you're in danger,
Speaker 1: it's normal for your blood pressure to rise. It's part
Speaker 1: of the sympathetic nervous systems fight or flight response. What's
Speaker 1: unusual in this case is the sudden decrease in blood
Speaker 1: pressure that causes you to lose consciousness. At the center
Speaker 1: of all this oddness is the vagus nerve. It's a
Speaker 1: major nerve that connects your brain to various regions of
Speaker 1: your body that are involved in involuntary movement like your
Speaker 1: heart beating, your throat swallowing, that kind of stuff. And
Speaker 1: at the other end, your vagus nerve is connected to
Speaker 1: a region of your brain called the nucleus of the
Speaker 1: solitary tract, or the NST for short. The n s
Speaker 1: T is kind of like a toggle switch that goes
Speaker 1: back and forth between the sympathetic response that's your fight
Speaker 1: or flight response, or the parasympathetic response, which is what
Speaker 1: calms you down after danger has passed. And what researchers
Speaker 1: think is going on is that the NST gets some
Speaker 1: sort of confused signal from the vagus nerve that causes
Speaker 1: it to decrease blood pressure as part of the parasympathetic
Speaker 1: response without deactivating the increase in your heart rate, which
Speaker 1: causes a lot of blood to suddenly be pumped away
Speaker 1: from your brain, hence making you pass out. Another explanation
Speaker 1: is that your NST simply toggles too quickly between the
Speaker 1: sympathetic and parasympathetic responses, like it's working like a joystick
Speaker 1: and it's going back up and down, and you your
Speaker 1: brain is saying what's going on? And then while you
Speaker 1: pass out on the floor. Then there's another parallel hypothesis
Speaker 1: that because you're n s T is also in charge
Speaker 1: of mediating your disgust response, that there's some sort of
Speaker 1: mixture of fright and disgust that causes you to faint,
Speaker 1: because again the NST is confused. However you slice it,
Speaker 1: it seems that you can lay the fainting at the
Speaker 1: sight of blood thing at the feet of the NST.
Speaker 1: Evolutionarily speaking, passing out at the sight of your own
Speaker 1: blood doesn't make much sense, and researchers have bent over
Speaker 1: backwards to try to explain it. What they've come up
Speaker 1: with is that possibly when you faint at the sight
Speaker 1: of your own blood, say, after being mauled by a bear,
Speaker 1: the bear will take you as being dead and maybe
Speaker 1: he'll lose interest. Well. Another more reasonable explanation is that
Speaker 1: this sudden decrease in blood pressure prevents us from bleeding
Speaker 1: out of some sort of wound, and the fainting is
Speaker 1: just an unfortunate byproduct of the whole thing. Either way,
Speaker 1: whatever the case, once you're on the floor, which is
Speaker 1: usually what happens when you faint, the blood flow to
Speaker 1: your brain can be restored fair really quickly, because it's
Speaker 1: a lot easier for your heart to pump blood horizontally
Speaker 1: than upwards against grab. Today's episode was produced by Tyler
Speaker 1: Clang and written by myself with style notes from Josh
Speaker 1: Clark for brain Stuff's Erstwhile YouTube series. If you like
Speaker 1: our show and also like not being naked, check out
Speaker 1: our March shop at t public dot com, slash brain stuff,
Speaker 1: and of course, for more on this and lots of
Speaker 1: other topics that attempt to explain the unexplained, visit our
Speaker 1: home planet how stuff Works dot com.