BrainStuff Classics: Why Do Some People Faint At The Sight of Blood?

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 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 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, 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 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 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 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 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 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 hypothesis that because you're n s T 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 maybe he'll lose interest. Well. Another more reasonable explanation is that this 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 fair really quickly, because it's a lot easier for your heart to pump blood horizontally than upwards against grab. Today's episode was produced by Tyler Clang and written by myself with style notes from Josh

Clark for brain Stuff's Erstwhile YouTube series. If you like our show and also like not being naked, check out our March shop at t public dot com, slash brain stuff, and of course, for more on this and lots of other topics that attempt to explain the unexplained, visit our home planet how stuff Works dot com.