system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When doing 30 minutes of cardio at a continuous pace, why does the heart rate significantly increase, and then start going back down before exercise is completed?
gpt
Your heart goes "oh fuck!" and starts pumping blood faster and faster. Then it realises it didn't need to be as high as it was, and it starts to slow down or level out.

Think of it like hitting the accelerator from zero to cruising speed.