The rheological properties of blood depend highly on the properties of its red blood cells: concentration, membrane elasticity and aggregation. These properties affect the viscosity of blood, as well as its shear thinning behavior. Moreover, it has been demonstrated that, in some cases, these properties are related to diseases, such as hemolytic anemias. By mean of the experimental analysis of the front advancement of blood in microchannels, we determine the viscosity of different samples of blood. We present a method that successfully scales the viscosity of blood, according to the concentration of erythrocytes and their biomechanical properties, bending elasticity and aggregation.