system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it unhealthy for your heart to race due to stress but healthy for it to race during cardio?

Is there a difference in the type of rapid heartbeat brought on by stress/panic compared to a rapid heartbeat as a result of exercise?
gpt
It's healthy during cardio because its usually a moderate increase in heartbeat, and as your heart rate goes up and your heart has to work harder, it becomes stronger. The stronger heart and the more "well-oiled" your vasculature gets from a strong heart, the better your health.

With panic/stress, it usually only lasts a brief time (seconds, maybe minutes) so your heart may be "working out" but its not gaining anything long term from its super short work out. Also, it's not necessarily unhealthy for your heart to race during stress. But it definitely can be, because its usually the result of chemical signaling which occurs really suddenly. One second your normal, the next you are freaked out with a racing heart. This huge jump in heart activity can cause arrhythmias and heart problems, and even result in chest pain and heart attacks. The same is true of cardio exercise, but since its more controlled and more of a "build up" than a sudden signal to work faster, its healthier and safer.