It comes like a wave over your body — that twist of the stomach, a rush of blood to the head, and then the ground seemingly spinning beneath your feet. And all because you’re seeing the red stuff streaming from an open wound. You may think that this trick is literally all in your head.

But reports that feeling faint at the sight of blood is actually a primeval reaction that goes back to our cave days. Dr. Fred Jaeger is medical director of the Center for Syncope and Autonomic Disorders at the Cleveland Clinic in Ohio. As he told, "Times of severe stress or injury or fear can trigger the reflex: your blood pressure drops, your heart rate slows." And therefore, you feel faint.

The condition is called vasovagal syncope. To experience it, a person is generally standing upright and witnessing something extremely unpleasant. The heart beats faster, sending the wrong message to the receptors in the heart's chambers that the ventricle is filled with blood. The brain gets the wrong message and slows down the flow of blood accordingly. Less oxygen gets to the brain and — viola, that person is kissing concrete.

Experts say this condition allowed our ancestors to self-preserve. As Jaeger explains to, “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.” And because you have tricked your adversary into letting you live, you therefore live to pass on your genes.

Six in 10 people are thought to possess this gene, but only an unlucky few will actually pass out because of it. Some experts suggest de-sensitization therapy to overcome the condition by looking at pictures of blood and guts. Others recommend deep, calm breathing to help the moment pass. The condition is not exclusive to humans, as possums and goats are known to become momentarily unresponsive when danger is near.

Ever wonder why some people faint at the sight of blood?
Feeling faint at the sight of red stuff is an evolutionary trick of self-protection.