The body responds to the demands you put on it.  If you lead a sedentary life the body responds by lowering your metabolic rate and letting your muscles atrophy.  If you lead a sedentary lifestyle and eat more calories than you burn you will put on weight.  On the other hand, if you task your body to work at a high heart rate or lift a weight in a way that stresses a certain muscle your body will respond as well.  When I train clients each exercise I choose is focused on creating a training adaptation.  We will repetitively stress specific systems of the body to gain the training adaptation necessary to meet your fitness goals.