MEDICAL PHYSIOLOGY

Detailed explanation-1: -During exercise, your body may need three or four times your normal cardiac output, because your muscles need more oxygen when you exert yourself. During exercise, your heart typically beats faster so that more blood gets out to your body.

Detailed explanation-2: -During heavy exercise, fast running, cycling, walking for many hours or heavy weight lifting, the demand for energy is high but the supply of oxygen to produce the energy is limited. So, the heart rate increases to supply more oxygen and glucose to the respiring cells.