system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to our bodies when someone makes us jump?
gpt
Your brain thinks there is an immediate danger and tells your body to suddenly release loads of adrenaline and activate a part of your nervous system called the sympathetic nervous system which deals with the fight or flight response. 

This has the effect of increasing your heart rate, making you more alert, making your hairs stand on end, breaking down energy stores so there's lots of glucose for muscles, dilating your pupils and a bunch of other effects. All of these effects are designed so that you can fight or run away as effectively as possible. 

When we realise that there isn't any danger, we downregulate all of the above effects.