Plasma is a Newtonian fluid because:
Viscosity is directly propoional to shear stress
Viscosity is equal to shear stress
Viscosity is inversely related to shear stress
Viscosity is independent of shear stress
For a Newtonian fluid, viscosity remains constant, independent of the shearing force. Whole blood is a non-Newtonian fluid; its viscosity goes changing with the changes in shearing force.
Blood Physiology