system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: the pit in the stomach, the physical sensation you get when something bad happens.

What exactly is this?  When you hear really bad news or see something terrible you get a physically uncomfortable feeling in your gut.  For instance my ex-wife is a drug addict and she called me the other day, as soon as I heard her slurring speech I got this physical drop in my stomach.  What is this, how is it useful, and how does it work?
gpt
When dealing with an unpleasant situation, your brain releases adrenaline. This is to prepare you to either confront a problem, or run from it. This happens very fast, and is the 'rush' sensation you feel when you trip or something similar.

The feeling in your stomach is one of the effects of this system: reduces the activity of your digestive system, in order to increase blood flow to the areas vital for immediate survival.

The part of your brain that triggers this can't actually observe reality itself, so it has to interpret it through other parts of your brain. Ultimately, it can't distinguish emotional pain from physical pain, and responds as if they were all real.