Article: Your Heart Keeps You Running

Mark kneels on one knee, his hands in front of him, ready to spring off the starting line and run the hundred-yard dash. "Bang!" The starting gun sounds, and Mark is off. He runs as fast as he can and crosses the finish line first. Right after the race, Mark's heart pounds so loud he feels as if everyone must be able to hear it. Before long, however, his pulse rate slows as he cools down from the excitement and effort of the race.

As he ran, Mark's heart was putting in extra effort to pump blood filled with energy-giving oxygen to his muscles. That's the heart's job. It's a hollow muscle about the size of your fist that pumps blood through your body to bring oxygen and nourishment to ...

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