system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does exercise help with so many health problems?

So many health issues like heart disease, kidney disease, liver disease... It seems to be the same case in that exercise should help keep those at bay. 

But why?

Doesn't exercise increase your heart rate? And isn't an elevated heart rate generally a not good thing outside of exercising?
gpt
The heart is a muscle, the stronger it is, the less pumps it needs to create a good blood flow.
People who do cardio will usually have a lower heart rate during normal activities due to this strength.