The mere sight of blood can make some of us feel light-headed. I’m getting a little breathless just writing this. For others, seeing blood can cause them to actually pass out. While it’s not serious, it can certainly be frightening. We’ve seen the trope in movies dozens of times — the man who claims to be a doctor only to pass out when his love interest cuts her finger. But why does this happen?

Well, it all comes down to vasovagal syncope. The vasovagal syncope trigger causes your heart rate and blood pressure to drop suddenly. That leads to reduced blood flow to your brain, causing you to briefly lose consciousness. It can be caused by the sight of blood or sudden emotional distress.

Evolution

Why would blood cause us sudden distress though? The answer may lie with our cavemen ancestors. It may have been a survival technique. Fred Jaeger, DO, medical director of the Center for Syncope and Autonomic Disorders at the Cleveland Clinic in Ohio, spoke to NBC News on the subject. “For example, if you’re a caveman and another caveman comes over and cuts your arm off, the sight of blood or injury may cause you to faint, so when you’re laying there on the ground, you’ll look like you’re dead to the other caveman and he won’t cut your head off.” Fainting at the sight of blood once had a purpose.

There may be a reason why women of yore were prone to fainting or swooning too. The vasovagal syncope is more common in women, as women tend to have a higher resting heart rate and lower blood pressure.

Look away

If you are the type of person who feels faint at the sight of blood, you just need to look away. It sounds obvious, but part of the sensation of vasovagal syncope can come from feeling out of control in the situation.

In a 2015 study, participants were divided into “perceived control” and “no perceived control” groups. They were shown a video of mitral valve surgery, which is known to trigger vasovagal syncope in non-medical personnel. The participants with no perceived control reported significantly more vasovagal symptoms and anxiety, compared with those who could take a break.

Signs to watch out for

Not sure you’re the type of person who faints at the sight of blood? We don’t recommend you prick your finger and see what happens. Watch out for these symptoms instead:

  • Feeling lightheaded or dizzy
  • Seeing spots or stars in front of your eyes
  • Have trouble breathing
  • Your fingertips and lips start to tingle
  • You feel hot suddenly
  • You feel nauseous
  • Your skin takes on a blueish tint

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