The heart is a muscle. It has one job, but that job is super important. Without it, we couldn't live. The heart pumps blood through our body. If you think of your body like a car, blood is the gasoline. We need fuel pumped so that our body can work. Blood carries nutrients and oxygen to all the parts of our body, and in carries waste away. When we exercise, we're making our bodies do more work than usual. Our muscles are stretching and flexing, and they need fuel to do that.
To help with the extra work, our heart starts working harder! They pump more quickly, and sends blood more quickly through our bodies. The rise in heartbeat during exercise is usually a very healthy thing!
You've probably heard it a lot: regular exercise is a very healthy thing. Next time, before you exercise, check your pulse. The pulse is your heartbeat. You can usually find it in your wrist (below your palm) or on the side of your neck (under your jaw). Use your middle and index fingers and press on those areas. You should be able to feel your pulse. Count how many heartbeats you can feel in ten seconds. Then exercise, and right after you stop, check your pulse again. How much has it gone up? How has that helped your body exercise?