The blood flow rate to the muscle will increase by up to 20 times the resting rate very quickly after the commencement of activity. In activities like swimming, cross-country skiing, or running, when the entire body and almost all skeletal muscles are active in some capacity, the cardiac output sent to the skeletal muscles reaches 80%. This high flow rate is necessary to provide the oxygen required by these muscles. At rest, the total blood volume is roughly equal to the total number of red blood cells plus the total amount of hemoglobin in your bloodstream. During exercise, you can expand blood volume by two ways: increasing the size of blood cells or adding plasma (the liquid component of blood). Exercise also increases red blood cell production rates in the bone marrow. During intense exercise, there is a large demand for oxygen, so the heart pumps more blood into the arteries. This increased blood flow fills the lungs with more oxygen-rich blood, and also flows into other parts of the body including the working muscles. The brain also requires a lot of oxygen during exercise, so some of this increased blood flow goes there too. The overall effect is that blood flow to all major organs increases during exercise. At first, the muscles of an inactive person can't use much oxygen, so they need plenty of blood flowing to them. Thus, at low levels of activity, the whole system is balanced - enough blood flows to all the muscles and other tissues so they don't suffer from lack of oxygen.