system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening to my body when I have a "sinking feeling?"

Whenever I see a police officer in my mirror, my heart will immediately skip a beat. I may even have temporary chest pain or a sinking feeling in my stomach. I might even start feeling warm. This is applicable to more scenarios than just this one. 

What is it that is happening that creates this feeling? Is it physical, or is it all mental? Does it mean anything?
gpt
It's mental *and* physical. Also: be aware that your brain *is* a part of your body, so anything that happens there is also "physical", actually.

It starts with a trigger that you get from the outside. For example you seeing the cops. Your brain recognizes this as a stress signal, as a signal of: "Oh man, trouble ahead". *Then*, your brain starts to influence the adrenal glands on top of the kidneys (via nerves) to produce adrenalin.

Here comes the physical part: Your body produces adrenalin. Your heart may or may not skip a beat, but it sure gets the signal to start beating faster and with more power. You start to breath heavier and quicker so that you'll get more oxygen in your blood. Blood goes to your head and your extremities (muscles) to make your ready for fighting or fleeing. Blood goes away from your stomach or intestines. It was there to help with digesting your food and taking up the nutrients, but right now that is not your main concern - you'll have to make do with the nutrients you've got, digestion will come later again, if you make it. In very scary situations, some people shit their pants or empty their bladder, which helps your body to become lighter and more able of running away or fighting. You start to sweat so that when your muscles will start to produce heat, they will not overheat (this will make you feel warm). 

All this is there to prepare your body to fight for your life, or to run for your life. In situations where this is necessary, this automatic reaction has saved lives, and lots of them. If people are in real danger, this helps them focus, and have the strength and stamina to do what they have to do. (People afterwards often say they didn't know they were that strong). You are here, because lots and lots of your ancestors have been in dangerous situations and the fight or flight reaction saved their ass... 

But if it is an inappropriate reaction, for example because you cannot fight or flee (for example *after* a car accident) or because there is nothing to fight with or flee from - this automatic reaction is rather useless. Still, we have it because of the other situations where it *is* useful.