What defines the viscosity of blood?

The viscosity of blood primarily depends on the proportion of cells to plasma, which is a measure of how concentrated the blood is with its cellular components, such as red blood cells, white blood cells, and platelets. When the number of cells increases relative to plasma, the blood becomes thicker and more viscous. This is particularly evident in conditions like polycythemia, where there is an increased concentration of red blood cells. The higher viscosity can affect blood flow and the heart's workload.

In contrast, factors like temperature can influence the viscosity temporarily, but they do not define it as clearly as the cellular-to-plasma ratio does. The presence of electrolytes has a peripheral effect on blood viscosity but is not a primary determining factor. While the color of blood may be an interesting property, it does not directly relate to blood viscosity, as it is more affected by oxygenation levels rather than the thickness of the blood itself. Therefore, the proportion of cells to plasma is the key determinant of blood viscosity.