When the person stop exercising, the body continue to use the aerobic energy, so it starts to have some physiological changes. The heart will gradually slow down (same as blood pressure), and breathing will gradually return to normal, since the body will not need more intake of oxygen then normal to stimulate a balance in energy and respiration, because of that the body will change the breathing pattern in order to get to the resting state, by using less oxygen. Therefore, the body uses oxygen to bring itself back to a normal, resting state.