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Your heart literally gets bigger! Your stroke volume goes up, and you probably have more blood plasma, too. This aids in fueling muscles.

Though, if you Google heart hypertrophy, you'll find a bunch of scary things, because your heart can also get bigger in a bad way rather than a good way.

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I'd guess there is also some efficiency in muscle tissue taking in oxygen over fat tissue? Also a bit better lungs, for runners and such? Though, I also don't know the relationship between lung efficiency and heart rate.

If you have links on any of this, it feels like it would be a very fascinating read.

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