If you faint at the sight of blood, you are not alone. It is estimated that 3.5 percent of people experience blood-injection-injury phobia during their lifetime – that is, an excessive and (at times) irrational fear of blood, injury, and/or injection.

Those of us who have a tendency to keel over when in the presence of blood have what is known as vasovagal syncope. "Vaso" is referring to the blood vessels and "vagus" a specific nerve that runs from the brain to the abdomen and plays an important role in regulating heart rate.

Having the condition means that a single drop of blood can cause your heart rate to crash. While this is going on, the blood vessels in your legs dilate so that blood pools in the legs and away from the head. Your blood pressure drops and you faint like a southern belle in an old black-and-white movie.

There are a few warning signs. Shortly before the faint, you might notice your skin pale, your vision blurring, and a feeling of light-headedness. Nausea, a cold and clammy sweat, tunnel vision, and warmth can also be symptoms, says the Mayo Clinic. 

And while for many people it is the sight of blood that causes this to happen, there are many, many other triggers, from the obvious (too much heat) to the peculiar (blowing the trumpet). Another common trigger is extreme emotional stress.

Medically speaking, blood-injection-injury phobia is (as it says on the tin) a phobia and like many phobias, it can be abated with exposure to the cause. However, unlike the vast majority of phobias, it prompts a decrease in heart rate rather than an increase in heart rate.

Why exactly this happens is a bit of a medical conundrum. There are several hypotheses (from the "adaptionist theory" to the disgust reflex), but one of the most persuasive arguments is the "the paleolithic-threat hypothesis", which is essentially a survival tactic stolen from opossums. 

The hypothesis is backed up by the fact that vasovagal syncope is far more common in women and youngsters than it is in men, with many boys "growing out" of their phobias during puberty. While it would hardly make sense for a Stone Age man to faint at the sight of blood – after all, they have mammoths to hunt and neighboring tribesmen to fight – it may have served as a biological form of protection for Stone Age women and children. Instead of being killed, for example, they may be passed over for dead or taken captive, a not great but preferable option to being murdered. 

Right now, this is just speculation. And in any case, it is probably not going to help all that much next time you have to go for a jab or blood donation.

While it cannot always be avoided, people who experience vasovagal syncope can lessen some of the effects by lying down and raising their legs when they notice those early signs, or (if lying down isn't an option) by sitting down with their head in between their knees. This allows gravity to help blood flow to the brain.