Why some people are afraid of blood
Actually, a lot of people are afraid of blood. The mere sight of it causes some people to get queasy and lightheaded and it all has to do with our evolutionary fear reflex.
When we see blood, our vagus nerve, which runs from the head to the heart, is triggered. It sends a signal that tells the heart to slow down, which is why we feel faint and sick. Seeing blood is unnatural and at first, we think it might be ours. When we realize it’s not, then our evolutionary instinct kicks in and we think there’s a predator out for us.
By fainting, we’re playing dead and/or slowing down the blood flow/volume to ensure that we don’t get attacked, and don’t lose too much blood if we are.
Those that see blood on the regular, however, get used to it and are able to manage that response.