What dawnrising mentions...the vasovagal reflex....my brother-in-law, and one of my daughter's boyfriends, had it.....my brother-in-law and daughter's boyfriend would have it whenever they caught a glimpse of their own blood....
Interestingly, they were both in professions that involved seeing other persons blood...a lot....
My brother-in-law is a dentist...and, my daughter's old boyfriend is a police detective, in a large city (lots of shootings)….
LOL....