system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is that sinking feeling in your stomach, blood warming throughout your body feeling you get when you hear bad news?
gpt
Your nervous system has two main modes, one is the well known "Fight/Flight" and the other is "rest and digest". 

When you've just eaten and feel safe, you want lots of blood flow through your gut to help absorb all those nutrients. You're in rest and digest mode.

If suddenly a tiger attacks you, the fight/flight mode kicks in, releases a bunch of adrenaline to get you going and tells your body that digesting food isn't as important as running away. 

Now we're not too good at unconsciously telling the difference between a real and perceived threat. When you get bad news, your body feels like there is a threat to it, so the fight/flight mode kicks in, blood flow to your gut is reduced (giving you that sinking feeling) and the adrenaline gets your heart pumping faster, warming you up a bit.