system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening biologically and psychologically when someone witnesses something so horrific they throw up?
gpt
Stress reaction.  

Stomach acid, bowel contents, bladder contents -- they all do really, really bad things to you if they end up outside of their designated organs.  Waaaay back in the day, if you saw someone getting mauled by a lion, there was a good chance you're next.  So your body then proceeds to evacuate those solids and fluids, so if you survive the mauling, you won't slowly die of sepsis.