system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some people pass out when seeing blood?
gpt
I don't know about particular theories related to blood or violence in particular, but the "fainting" reaction related to all stressful situations is called a "vaso-vagal reaction." To some, a stressful situation is seeing blood or violence. The stress overstimulates the nucleus tractus solitarii in the brainstem. This in turn amplifies the parasympathetic system, in other words your rest and digest nervous system. This causes cardio inhibition and dilation of your blood vessels, which causes blood pressure to bottom out as a result. In turn, you faint. Ask if you have any more questions about the physiology of this process.