Phobia

Here's Why Fear Of Blood Makes You Faint When Other Phobias Make Your Heart Race

When people with blood injury injection (BII) phobia encounter their fears—which include seeing blood, anticipating an injury, or getting a shot—their heart rate and blood pressure drop, sometimes so much that they faint.

What's Going On

When faced with a fear, your body tends to jump into action, sending your heart racing and your blood pressure into the stratosphere. This is true for all phobias, whether it's a fear of heights or spiders or clowns. All, that is, except for one: BII phobia.

The difference is that most phobia sufferers experience what's called a phobic reaction, which starts in the brain and moves into the sympathetic nervous system as a part of the body's fight-or-flight response. Along with a spike in heart rate and blood pressure, the response can cause sweating, trembling, and tense muscles. Suffers of BII phobia, on the other hand, experience what's called a vasovagal response, triggered by the vagus nerve as part of the parasympathetic nervous system. When that nerve goes haywire, it causes a drop in heart rate and blood pressure that in turn causes dizziness, sweating, tunnel vision, nausea, and fainting.

So why does this happen? There isn't a clear answer, but plenty of experts have ventured a guess. Some researchers think that fainting at the sign of blood is an evolutionary leftover from the instinct to play dead in the presence of danger. Others point to a different evolutionary origin: if our ancestors were stabbed by a spear or mauled by a lion, a drop in blood pressure would have helped keep blood loss to a minimum and ensure their future survival. Despite the evolutionary uncertainty, one thing is pretty clear: BII phobia is probably genetic. More than 60 percent of sufferers have a first-degree relative with the phobia, and studies have shown that identical twins are more likely to share the phobia than fraternal twins.

Smart Graphic

Why It Matters

This is no obscure phenomenon: BII phobia affects three to four percent of the general population, which translates to nearly 13 million people in the U.S. alone. That's a lot of extra difficulty when it comes to people cutting themselves in the kitchen or getting a flu shot. Luckily, science says BII phobia can be cured: a 1991 study showed that after five sessions of being exposed to their phobia trigger while tensing muscles in their arms, torso, and legs, up to 90 percent of subjects stopped experiencing the majority of their symptoms.

Editors' Picks: Our Favorite Videos About Phobias And Fainting

Why Do Some People Faint At The Sight Of Blood?

Hear what the experts think.

What Happens When You Faint?

It all starts in the vagus nerve.

Share the knowledge!

Is Your Fear A Real Phobia?

Just because a fear has a name doesn't mean psychiatrists agree it's a real phobia.

Share the knowledge!