Relationships between hematocrit, viscosity, and shear rate are important factors to put into consideration. Since blood is non-Newtonian, the viscosity of the blood is in relation to the hematocrit, and as a function of shear rate. This is important when it comes to determining shear force, since a lower hematocrit level indicates that there is a need for more force to push the red blood cells through the system. This is because shear rate is defined as the rate to which adjacent layers of fluid move in respect to each other.  Plasma is a more viscous material than typically red blood cells [ citation needed ] , since they are able to adjust their size to the radius of a tube; the shear rate is purely dependent on the amount of red blood cells being forced in a vessel.