Modelling of blood flow on the micrometer scale is a challenging task due to its non-Newtonian behavior, and because of the small dimensions which makes the continuum assumption problematic. The problem can be approached by the theory of generalized Newtonian fluids, where the apparent viscosity is described as a function of the scalar invariants of the shear rate tensor. The present report contains a review on flow and rheological properties of blood. In this work the nonlinear character of the constitutive equation has been measured using a rotational rheometer. The measurements have been fitted to a simple power law model, and used as a constitutive equation for CFD-simulations in simple microtubes. By means of a parallel plate geometry, it was the intention to investigate how the viscosity depends on the gap size between the shearing plates. However, due to the instrument it was difficult to obtain reliable results at small gap sizes.