How your heart works......Your heart is about the size of your two fists. It pumps about 5 or 6 quarts of blood per minute through out your body.
When you exercise your muscles require more oxygen and nutrients at a quicker rate to fuel contractions. The natural response to exercise is that your heart will beat faster to deliver more blood. All the arteries and capillaries dilate to accommodate the increased blood flow to the muscles and extremities. In contrast, blood vessels will constrict in the areas of your body that is less active during that type of exercise.