Why do you breathe heavy when you exercise? Why does your heart beat faster when you exercise?

Your body's cells are consuming energy faster, particularly oxygen. Your heart and breathing rate speed up to compensate.

When you exercise, your body's cells perform their duties at a faster pace than usual. As such, they require more resources - oxygen and fuel - to keep running smoothly.

You will begin breathing heavily to not only intake greater amounts of oxygen but to expel the larger amounts of carbon dioxide that are being produced. Your heart rate goes up in order to deliver the oxygen to the cells that need it and to remove the carbon dioxide from the body.