This is called "vasovagal syncope". [1] Certain emotional triggers, including the sight of blood, cause the brain to command the cardiovascular system to lower blood pressure. In some people this reflex is strong enough to cause loss of consciousness due to the decreased blood pressure.
There's a number of competing theories as to why this happens, possibly an evolved response to help control bleeding or vestigial tonic immobility (feigning death to hide from predators). [2] Either way, it's kinda fascinating.
There's a number of competing theories as to why this happens, possibly an evolved response to help control bleeding or vestigial tonic immobility (feigning death to hide from predators). [2] Either way, it's kinda fascinating.
[1]: https://en.wikipedia.org/wiki/Reflex_syncope#Vasovagal
[2]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5135249/