Viscosity is a propety of all matter. It is most often used to describe the differences in liquids. Because blood is a liquid, it is no exception.
The normal range of viscosity in your blood is very narrow. If viscocity is defined as the resistance to flow where high viscosity means it is highly resistance to flowing (like sap from an evergreen tree) then it would mean that something has caused your blood to thicken up. Your blood can also become too thin (from using blood thinning medicine like aspirin, for example). This cause the blood to have a lower than normal viscosity.
When you are exercising the importance of blood is to quickly and efficiently bring oxygen to your organ's cells and remove carbon dioxide from the cells as a result of cellular respiration and the need for extra energy during exertion. If the blood were at the thick range of normal (higher viscosity) it would not be as efficient at doing its job under pressure. Therefore, during exercise the body emits chemicals that cause th viscosity of the blood to lower so that it may deliver an increased amount of oxygen to the necessary muscles and organs. The lower viscoity allows the blood to "flow" through the ateries and veins in a more efficient manner.
Keep in mind, we are working within a narrow range of "normal" values here. The difference between blood that is flowing during exertion and blood that flows at rest would not be drastic. It would be like the difference between rubbing alcohol and water, not molasses and water.
The flow of blood through the vessels of the circulatory system is a function of the pressure in the system and the resistance to flow caused by the blood vessels. Blood flow is directly proportional to pressure and inversely proportional to resistance.
that is Blood flow= pressure/resistance
If the pressure in a vessel increases then the blood flow will increase. However, if the resistance in a vessel increases then the blood flow will decrease. Resistance in the blood vessels is effected by three parameters:
1. Length of the vessel. The longer the vessel the greater the resistance.
2. Viscosity of the blood. The greater the viscosity the greater the resistance.
3. Radius of the vessel. The smaller the radius the greater the resistance.
When you exercise, blood flow to the skeletal muscles increases significantly (up to 80-85% of total blood flow).So, viscosity decreases.
Consequently, the blood flow to a resting muscle is LOW, that means, viscosity is HIGH.