Discuss as:

Why some people faint at the sight of blood

Writes Bill Briggs: Ever grabbed some floor after glimpsing some blood?

Does a mere peek at the red stuff – yours or anybody’s – make you feel woozy and wobbly?

If so, you can thank your Uncle Caveman or Aunt Cavewoman.

This reaction is primeval stuff, buried deep within our brains. It goes by the name "vasovagal syncope." More importantly, medical experts believe fainting at the sight of blood probably helped some of our ancestors survive some horrible things. (So stop teasing folks who take a quick trip to Dreamland whenever they spot the tiniest trickle of the vital fluid: We may owe them our very existence.)

While only a portion of humans experience the phenomenon, “it’s a reflex that’s built into every person on this planet,” says Dr. Fred Jaeger, medical director of the Center for Syncope and Autonomic Disorders at the Cleveland Clinic in Ohio. He’s studied these bewildering blackouts for 26 years.

“Times of severe stress or injury or fear can trigger the reflex: your blood pressure drops, your heart rate slows,” Jaeger says. Most medical experts speculate that such fainting spells are likely rooted in evolution – an ancient instinct that somehow kept a foothold in our DNA as we spent eons maturing.

“For example, if you’re a caveman and another caveman man comes over and cuts your arm off, the sight of blood or injury may cause you to faint,” Jaeger says. “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.

“This goes back to the time when we were hunter-gatherers, warriors, Cro-Magnon people,” he adds “Vasovagal syncope probably had some benefit – if you were injured and you lived to fight another day, the gene would be carried on. Just like survival of the fittest.”

Another bonus for those early humans who keeled over at first blood: their blood pressures dips and heart rate drops also slowed their bleeding, which helped the stay alive.

While doctors believe the reaction is unique to humans and primates, some animals – like possums and a certain breed of goats – are known to swoon and become momentarily unresponsive when peril is present.

About six in 10 people will faint at least once due to this strange syncope. Common triggers include pain, standing for long periods or standing suddenly after a big meal. When the sight of blood causes the collapse, doctors dub that “phobic fainting.” For people who suffer chronic, fear-based fainting spells, (like going to the dentist) doctors will prescribe medications that work with their brains or nervous systems to keep them upright.

But why do an unfortunate few go horizontal each time they catch a flash of blood?

Researchers are analyzing genes and hormone levels. They’re hunting for clues in enzyme deficiencies.

“What is it in the makeup of easy fainters? We’ve got to suspect they just have this gene that is very easily triggered,” Jaeger says. “But that is the million-dollar question.”

Do you faint at the sight of blood? Tell us about it.

Find The Body Odd on Twitter and on Facebook.