Why does your pulse rate increase with exercise?

Becouse during the execise, the muscles need energy for doing their job. This energy is obtained by carbohydrate or fat burning, and for this reaction oxygen is needed. The more intensive the execise is, more quantity of oxygen it needs. For carrying more oxygen to the muscle, the heart needs to increase the number of beats per minit. This way more blood (carrying oxygen and carbohydrates) can be transported to the muscle.