Viscosity is defined as the ability of a fluid to resist flow. Viscosity of the blood is increased if the hematocrit is increased. Hematocrit is defined as the proportion of red blood cells to the total volume of blood. Hematocrit can be increased by conditions such as polycythemia vera (increased production of red blood cells) and dehydration (decrease in plasma volume).
Viscosity of the blood is determined by the hematocrit, red blood cell aggregation and deformability, and plasma viscosity with the hematocrit being the most important.