One-dimensional models of the systemic arterial tree are useful tools for studying wave propagation phenomena, however, their formulation for frictional losses is approximate and often based on solutions for developed flow in straight non-tapered arterial segments. Thus, losses due to bifurcations, tortuosity, non-planarity and complex geometry effects cannot be accounted for in 1-D models. This may lead to errors in the estimation of mean pressure. To evaluate these errors, we simulated steady flow in a patient specific model of the entire systemic circulation using a standard CFD code with Newtonian and non-Newtonian blood properties and compared the pressure evolution along three principal and representative arterial pathlines with the prediction of mean pressure, as given by the 1-D model. Pressure drop computed from aortic root up to iliac bifurcation and to distal brachial is less than 1mmHg and 1-D model predictions agree well with the 3-D model. In smaller vessels like the precerebral and cerebral arteries, the losses are higher (mean pressure drop over 10mmHg from mean aortic pressure) and are consistently underestimated by the 1-D model. Complex flow patterns resulting from tortuosity, non-planarity and branching yield shear stresses, which are higher than the ones predicted by the 1-D model. In consequence, the 1-D model overestimates mean pressure in peripheral arteries and especially in the cerebral circulation.