1 00:00:02,040 --> 00:00:10,000 Speaker 1: Welcome to Brain Stuff from How Stuff Works. Hey, welcome 2 00:00:10,039 --> 00:00:12,520 Speaker 1: to Brain Stuff. I'm Christian Sager, and this is the 3 00:00:12,560 --> 00:00:15,360 Speaker 1: episode where I explain to you why some people faint 4 00:00:15,440 --> 00:00:17,880 Speaker 1: at the sight of their own blood. Fainting at the 5 00:00:17,920 --> 00:00:20,720 Speaker 1: side of blood, which is either a condition called neuro 6 00:00:20,800 --> 00:00:27,040 Speaker 1: cardiovascular syncope or vasovagil syncope, is actually related in some 7 00:00:27,160 --> 00:00:31,360 Speaker 1: cases to what's classified as a blood injury phobia. Something 8 00:00:31,400 --> 00:00:33,920 Speaker 1: like three to four percent of people have a blood 9 00:00:33,960 --> 00:00:37,839 Speaker 1: injury phobia. But what's really interesting is that percent of 10 00:00:37,880 --> 00:00:40,840 Speaker 1: people faint at the sight of blood, which means there's 11 00:00:40,840 --> 00:00:43,120 Speaker 1: a lot of people out there who really have no 12 00:00:43,240 --> 00:00:46,560 Speaker 1: issue with cutting themselves, but they still faint dead away 13 00:00:46,960 --> 00:00:52,040 Speaker 1: any time they see themselves bleeding. That's kind of weird, right. Well, 14 00:00:52,120 --> 00:00:54,920 Speaker 1: when you faint from anxiety, which is what researchers think 15 00:00:55,040 --> 00:00:57,000 Speaker 1: is going on, when you faint from the side of 16 00:00:57,040 --> 00:01:00,800 Speaker 1: your own blood, your blood pressure suddenly spite, but then 17 00:01:01,240 --> 00:01:04,880 Speaker 1: just as quickly it decreases, and that decrease in blood 18 00:01:04,880 --> 00:01:08,039 Speaker 1: pressure drains blood away from your brain, causing you to 19 00:01:08,080 --> 00:01:11,520 Speaker 1: lose consciousness. When you're anxious or when you feel like 20 00:01:11,560 --> 00:01:14,360 Speaker 1: you're in danger, it's normal for your blood pressure to rise. 21 00:01:14,800 --> 00:01:18,800 Speaker 1: It's part of the sympathetic nervous systems fight or flight response. 22 00:01:19,319 --> 00:01:22,520 Speaker 1: What's unusual in this case is the sudden decrease in 23 00:01:22,560 --> 00:01:26,760 Speaker 1: blood pressure that causes you to lose consciousness. At the 24 00:01:26,800 --> 00:01:30,000 Speaker 1: center of all this oddness is the vagus nerve. It's 25 00:01:30,000 --> 00:01:33,160 Speaker 1: a major nerve that connects your brain to various regions 26 00:01:33,160 --> 00:01:37,320 Speaker 1: of your body that are involved in involuntary movement like 27 00:01:37,440 --> 00:01:40,520 Speaker 1: your heart beating, your throat swallowing, that kind of stuff. 28 00:01:40,840 --> 00:01:43,840 Speaker 1: And at the other end, your vagus nerve is connected 29 00:01:43,840 --> 00:01:46,280 Speaker 1: to a region of your brain called the nucleus of 30 00:01:46,360 --> 00:01:50,720 Speaker 1: the solitary tract or the NST for short. The n 31 00:01:50,760 --> 00:01:53,000 Speaker 1: s T is kind of like a toggle switch that 32 00:01:53,040 --> 00:01:56,440 Speaker 1: goes back and forth between the sympathetic response that's your 33 00:01:56,440 --> 00:02:00,760 Speaker 1: fight or flight response, or the parasympathetic response, which is 34 00:02:00,760 --> 00:02:03,960 Speaker 1: what calms you down after danger has passed. And what 35 00:02:04,040 --> 00:02:06,880 Speaker 1: researchers think is going on is that the NST gets 36 00:02:06,920 --> 00:02:09,800 Speaker 1: some sort of confused signal from the vagus nerve that 37 00:02:09,880 --> 00:02:13,080 Speaker 1: causes it to decrease blood pressure as part of the 38 00:02:13,120 --> 00:02:17,720 Speaker 1: parasympathetic response without deactivating the increase in your heart rate, 39 00:02:18,120 --> 00:02:21,200 Speaker 1: which causes a lot of blood to suddenly be pumped 40 00:02:21,200 --> 00:02:25,079 Speaker 1: away from your brain, hence making you pass out. Another 41 00:02:25,200 --> 00:02:29,639 Speaker 1: explanation is that your NST simply toggles too quickly between 42 00:02:29,680 --> 00:02:33,520 Speaker 1: the sympathetic and parasympathetic responses, like it's working like a 43 00:02:33,600 --> 00:02:36,600 Speaker 1: joystick and it's going back up and down, and you 44 00:02:36,600 --> 00:02:39,880 Speaker 1: your brain is saying what's going on? And then while 45 00:02:39,919 --> 00:02:43,000 Speaker 1: you pass out on the floor. Then there's another parallel 46 00:02:43,080 --> 00:02:47,000 Speaker 1: hypothesis that, because your NST is also in charge of 47 00:02:47,160 --> 00:02:51,160 Speaker 1: mediating your disgust response, that there's some sort of mixture 48 00:02:51,240 --> 00:02:54,600 Speaker 1: of fright and disgust that causes you to faint, because 49 00:02:54,919 --> 00:02:58,480 Speaker 1: again the NST is confused. However you slice it, it 50 00:02:58,560 --> 00:03:00,799 Speaker 1: seems that you can lay the fainting at the sight 51 00:03:00,880 --> 00:03:05,359 Speaker 1: of blood thing at the feet of the NST. Evolutionarily speaking, 52 00:03:05,600 --> 00:03:07,880 Speaker 1: passing out at the sight of your own blood doesn't 53 00:03:08,080 --> 00:03:11,600 Speaker 1: make much sense, and researchers have bent over backwards to 54 00:03:11,639 --> 00:03:14,040 Speaker 1: try to explain it. What they've come up with is 55 00:03:14,080 --> 00:03:16,160 Speaker 1: that possibly when you faint at the sight of your 56 00:03:16,160 --> 00:03:19,280 Speaker 1: own blood, say after being mauled by a bear, the 57 00:03:19,400 --> 00:03:22,359 Speaker 1: bear will take you as being dead and maybe he'll 58 00:03:22,400 --> 00:03:27,200 Speaker 1: lose interest. Well. Another more reasonable explanation is that this 59 00:03:27,360 --> 00:03:30,800 Speaker 1: sudden decrease in blood pressure prevents us from bleeding out 60 00:03:31,240 --> 00:03:34,080 Speaker 1: of some sort of wound, and the fainting is just 61 00:03:34,160 --> 00:03:38,520 Speaker 1: an unfortunate byproduct of the whole thing. Either way, whatever 62 00:03:38,560 --> 00:03:41,320 Speaker 1: the case, once you're on the floor, which is usually 63 00:03:41,400 --> 00:03:43,840 Speaker 1: what happens when you faint, the blood flow to your 64 00:03:43,840 --> 00:03:47,560 Speaker 1: brain can be restored fairly quickly because it's a lot 65 00:03:47,680 --> 00:03:51,600 Speaker 1: easier for your heart to pump blood horizontally than upwards 66 00:03:51,760 --> 00:03:59,400 Speaker 1: against graftic check out the brain stuff channel on YouTube, 67 00:03:59,480 --> 00:04:01,640 Speaker 1: and for more on this and thousands of other topics, 68 00:04:01,840 --> 00:04:17,039 Speaker 1: visit how stuff works dot com.