The Real Reason People Faint When They See Blood

Updated: Apr. 13, 2021

The good news is that it’s probably not serious, and recovery is usually quick.

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Watching a scary movie with blood and gore can make some people’s skin crawl, but for others, even the possibility of real-life exposure to blood is a problem. The sight of blood or a needle, or sometimes even just hearing a story about it, can make these people pass out. We see this phenomenon illustrated in movies and television shows all the time, such as when a woman goes into labor and her husband is of no help and just faints to the floor.

When you faint at the sight of blood, you’re experiencing vasovagal syncope, which makes your heart rate and blood pressure decrease drastically. When your blood pressure drops so suddenly, the lessened blood flow to your brain causes you to faint.

Unless you show signs of having chronic low blood pressure, fainting from the vasovagal response is not something to worry about or a reason to call 911. There are also reasons that even healthy people faint that might not include seeing blood. If your fainting is random, however, you may want to see a doctor to make sure it is not a sign of a more serious condition. Here are some other reasons that people faint.

Another reason people faint at the sight of blood could be an inherited trait. According to Fred Jaeger, DO, medical director of the Center for Syncope and Autonomic Disorders at the Cleveland Clinic in Ohio, fainting might be a response passed down from our caveman ancestors.

“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,” Dr. Jaeger told NBC News. “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.” Dr. Jaeger points out that vasovagal syncope may have been a trait passed down by those who lived to fight another day—a perfect example of survival of the fittest.

Once you know the clear signs that you’re about to faint, you can do things like lie down or sit and prop your legs up. This helps gravity keep the blood flowing to your brain. To recover quickly after fainting, use the applied tension technique to increase your blood flow again.

Next, find out the silent signs that you might have low blood pressure.