When you exercise, your muscle cells require more oxygen for respiration so your breathing rate goes up to increase the amount of oxygen taken in by the body and to also expel the byproduct of respiration, carbon dioxide at a faster rate. Your heart rate also increases when exercising because your heart pumps oxygen rich blood around the body and gets it to where it needs to go ie the muscle cells. A faster breathing rate and a higher heart rate happen as a result of homeostasis, our muscle cells are deprived of oxygen and our body wants to restore oxygen levels in our cells.