This internal friction contributes to the resistance to flow. Whole blood has a relative viscosity of 3-4 depending upon hematocrit, temperature, and flow rate. Hematocrit is an important determinant of the viscosity of blood.
Temperature also has a significant effect on viscosity.
As temperature decreases, viscosity increases. This effect has several implications. The use of whole body hypothermia during certain surgical procedures also increases blood viscosity and therefore increases resistance to blood flow. The flow rate of blood also effects viscosity. At very low flow states in the microcirculation as occurs during circulatory shock, the blood viscosity can increase quite significantly.