system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If your heart has roughly 1 billion beats before natural failure, why is cardio exersize good for you?

Is it the same principle behind other muscles? Working them out makes them stronger?
gpt
Two reasons.

- Cardio doesn't just exercise your heart. It exercises your lungs and your muscles associated with endurance. This helps your body a LOT, doing things like assisting your metabolism to keep your weight down, adding to your bone density from the impacts, lowering your blood pressure, and increasing your stability so you won't fall and break a hip. 

- And your heart has *roughly* a billion beats, there's no ticking countdown clock. Some people get way more, some people get way less. It's more of a rough general observation than any sort of limiting estimate or warranty period.

P.S. fit people have lower beats per minute than unfit people, offsetting the extra heartbeats that occur when they exercise anyway.

*LATE EDIT: A lot of people have already pointed out that the average lifespan of a human means closer to 3 billion beats. Just pushing that up to the top for accuracy.*