Parasympathetic nervous system
However, in a healthy person, the brain realizes when the parasympathetic nervous system's effects are getting too strong.  For example, the brain may realize that the person is breathing too slowly, or that their heart rate is too low.  The brain reacts by making the sympathetic nervous system kick in.  Because the sympathetic nervous system has the opposite effects, it will make the person breathe faster and raise their heart rate.  These two systems have to balance each other constantly for a person to stay healthy.