system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why is making your heart work harder good for it?

(running/working out)
gpt
Your heart also has muscles, just like in your arms and legs. Making it work harder for a little while helps to build the strength of the heart. In turn, the heart is able to pump harder thus supplying oxygen to your body more efficiently. It's also able to pump fewer times to get the same amount of oxygenated blood to your body. All this means that even when you're not exercising, your heart doesn't have to work as hard.