Step-by-step explanation:
When you exercise the muscles in your body must contract, in order to do that they need oxygen, glucose, a molecule called ATP, and amino acids. As your muscles use these compounds and contract themselves, they will create waste products like carbon dioxide, and lactic acid that must be carried away from the muscles. When exercising many muscles will all require nutrients and elimination of waste products constantly at the same time. To meet this demand the heart must rapidly increase the rate at which it beats and pushes blood through the body. This is why the heart beats significantly faster during exercise.