aprillee,
Fainting at the sight of blood (or in other exciting/frightening events) generally comes from an overactive vasovagal response, an evolutionary fear reflex. This response slows down your heart rate and lowers your blood pressure, causing blood to drain to your legs. This means that less oxygen-rich blood is going to your brain, which is what causes people to feel lightheaded, or even pass out.
This survival mechanism is nice if, say, you need to play dead in the presence of a predator, which may be the origin of the response. And if youâre bleeding, the reduced heart rate might help prevent too much blood loss. But in most situationsâespecially emergencies when you need to be, well, awakeâitâs a nuisance.
This reflex is present in all humans. Some are just more sensitive to it than others. You can go through a desensatization process and reduce the effects.
Hope that helps some. Good luck.