The body is an amazingly complex machine. At any given time there is intricate communication among various cells, tissues, organs, and systems that serve to coordinate its physiological functions. When you think of the numerous processes occurring within the body at any given time, it is truly remarkable that these physiological functions work so well together. Even at rest, the body is physiologically quite active. Imagine, then, how much more active all of these body systems become when you engage in exercise. During exercise, nerves excite muscles to contract. Exercising muscles are metabolically active and require more nutrients, more oxygen, and efficient clearance of waste products. How does the whole body respond to the increased physiological demands of exercise?