system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when something really hurts our feelings we can feel it in the pit of our stomach and chest?
gpt
Your body and brain and part of the same system. Your emotions impact your body, just like your body impact your emotions. 

A section of your nervous system lines your digestive tract: the enteric nervous system. It is full of motor neurons that contract and relax muscles all the way from the esophagus to the anus to get your food into your stomach, digested, and eliminated (among other functions). Normally you wouldn't notice it working away, but if things go into overdrive, your stomach can feel weird.

[There is quite a bit of overlap between nervous system and the gastrointestinal system. Neurogastroenterology is the focus of a lot of research right now because there is a lot we simply don't know.]

Anxiety or stress causes muscle tightness all over the body--it readies the body for fight, flight, or freezing. There are muscles all over your chest, so you feel it as tension or tightness. This response also increases blood pressure, heart rate, breathing, and other functions. The result is that you can feel like your chest is heavy, feel your heart racing, feel your heart pounding, feel shortness of breath, etc.