Your body is nourished, cleansed and maintained by your blood and lymph systems. Pumped by your heart, your blood circulates oxygen and nutrients to your muscles, where they are converted in to energy. A by product of this process is lactic acid, which is filtered out with toxic wastes and carried away by your lymph system - the same miraculous 'sanitary system' that is in charge of your immunity, protecting you from disease. When activity becomes strenuous, however, this process looses its efficiency. Energy requirements exceed supply, and your feel it when the force of your swing at a tennis ball, for instance, isn't as hard after two hours of play as it was when you started. A similar problem occurs with too little activity - when you're at your desk too long and find your mind slower, your irritation higher, and tension grabbing your neck and shoulders.