Red blood cells live for about 120 days from the time they are formed in the bone marrow to the time they are removed by the spleen. During those 4 months, red blood cells grow progressively stiffer and more rigid. This increases the viscosity of blood. Red blood cell stiffness is the second most influential factor that determines a persons blood viscosity . Blood donation removes a portion of old and stiff red blood cells, replacing them with softer, younger cells.
Basically your trying to reduce the viscosity of your blood.