system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is that sinking/sickly feeling you get in your chest when thinking of losing something dear/going through a breakup?

Title says it all, we all know the feeling but why do we physically feel the pain in our bodies?
gpt
That feeling in your stomach is caused by anxiety.  Anxiety triggers the ancient “Fight or Flight” response that is programmed in to your DNA.  Anxiety manifests itself in complicated ways.  Here are a few:


When you feel stressed or anxious, your body releases a rush of hormones. Neurotransmitters in the brain react by sending messages to the rest of your body to:

1) Get the heart pumping faster
2) Increase the breathing rate
3) Tense the muscles of you abdomen 
4) Send more blood to your brain

Anxiety and stress can affect virtually every body system. This includes your cardiovascular, endocrine, musculoskeletal, nervous, reproductive, and respiratory systems.  In the digestive system, anxiety can cause:

1) nausea, vomiting
2) heartburn, acid reflux
3) stomachache, gas, bloating
4) diarrhea, constipation, painful spasms in the bowel

All of these symptoms together create the feeling of “Butterflies” in your stomach.