The human body adapts in many ways to regular exercise. That's why it gets easier to run three miles the more you train. When you exercise, your heart pumps oxygen-rich blood to the active muscles, giving them fluids and nutrients, and then drains the waste products away. The more blood that is pumped, the more oxygen is available to the working muscles. As muscles are worked over time, they become better at using the oxygen in the blood to produce more energy. An untrained person pumps much less blood during exercise than a trained athlete. Trained athletes have stronger heart muscles, which can contract more and hence pump more blood with each stroke.
In the animation: A runner's muscles stretch, heart rate rises, temperature goes up, breathing quickens, digestion slows down, muscles burn sugar and muscles produce lactic acid.
The heart adapts differently in response to weight training. In this case, the heart must generate more force with each beat to pump blood. It needs to overcome the pressure in the blood vessels caused by muscle contraction. The heart walls thicken in response to weight training, so that it can generate more force.
The circulatory system also adapts when you exercise. Less blood flows to major organs -- except for the heart and brain -- and more flows to the working muscles and skin. When you are at rest, 20 percent of your blood flows to the muscles, compared to 88 percent at maximum exertion. Arteries and veins in the muscles constrict and dilate during exercise to adapt blood flow and distribution to your body's needs.