Answer:
When exercising, the heart rate rises due to an increase in the amount of oxygen required by the muscles.
Step-by-step explanation:
Muscles require oxygenated blood, and as someone works out, the muscles require more of it to maintain adequate energy to keep up with the demand. If you exercise on a hot day, the combination of the high temperature and the strenuous activity performed by your heart will cause an unnatural increase in your heart rate. This occurs because the heart must pump blood to the skin to cool you down as well as deliver blood to working muscles.