Being able to accurately reproduce the physiological phenomena which take place in blood vessels requires understanding the impact physical parameters have on the fluid’s flow. Modeling blood flow implies choosing, or defining, at least one model with the corresponding parameters within the numerous possibilities which exist. The choice between Newtonian and non- Newtonian models is important and within these families there are many possibilities. The Navier-Stokes equations are solved using two models, a classical Newtonian model and the Carreau model. First some validation cases are presented to ensure the implementation is correct, then a patient specific geometry is studied to compare the differences both models induce. The results emphasize that the impact the different models have on the flow appears for the lower end of the velocity range which take place in the aorta. Highly unsteady blood flow behaves like a Newtonian fluid, whilst the quasi-steady flow presents noticeable differences.