Why do some people faint when they see blood? what causes the fainting?

Vasovagal reaction: Autonomic nervous system is hypersensitive and fainting is triggered. The reaction results in low blood pressure, and secondary collapse. This can be de-conditioned, and some meds may also help.

Vasovagal Reaction: This is fairly common and an interesting and generally well understood reaction. When some people see blood or other things that shock them, there blood pressure or heart rate goes up from catecholamine (adrenaline) release and the body then tries to correct this and can sometimes overshoot causing a drop in the HR and/or BP. Biofeedback training, therapy and some medications can help cure this,