Josh explains the science of why people faint at the sight of blood.

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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.

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