system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] What actually happens when we feel our heart “pounding”? Does it physically pound harder, does our heart rate increase, or do we just feel it and nothing physically is happening?
gpt
There are a number of possibilities:

Your heart might be beating faster.  Your body is pretty good at ignoring things that remain constant (when was the last time you noticed the feeling of the shirt you are wearing?) Your heart rate generally stays within a range, and when it beats much faster than that you become aware of it because it is different than what your body is accustomed to.

It may be beating harder.  If you have an adrenaline rush, or you are dehydrated, or are exerting yourself your heart not only beats faster, but also harder.  Again, your body is used to your heart beating fairly consistently, so if it deviates too far it becomes noticeable.

You may be more attuned to your own heartbeat.  If you are anxious, or if you are in a setting with minimal sensory input (dark, quiet) your body may be able to focus more attention on things that are usually ignored, such as your heart beat.

It could be a combination of any of these.

Edit to add:  could also be a ‘skipped beat,’ causing a single beat with increased filling of the heart, which makes the heart become temporarily more full, subsequently requiring the heart to beat harder on the next beat to push out that increased volume.