There a lot of hormones involved when we're in the fight-or-flight mode and they have a complex relationship to the other hormones in our body, which would probably require separate article(s) of their own lol.

I'm not a medical student but think of it as the person gets more emotionally acclimatized to doing more and more dangerous things, until is escalates and the body hits its physical limit, leading to a heart attack or something.

However, our genes don't change permanently...eventually the body can revert back to normal especially if there's reliable relaxation.