The governing equations defining the separation of blood in continuous flow centrifuges are derived using an empirical model for the viscosity of blood. The effects of fluid shear on the separation process are addressed and further models are developed to permit estimations of shear rate during centrifugation. Simplified predictive equations for species specific separations are developed. Contributions due to shear enhanced diffusion are addressed and found to be negligible at the discontinuous interface between cells and plasma. Experimental results from an investigational centrifuge are presented and compared with theoretical predictions. The role of rouleaux formation in conventional centrifuges is discussed and known centrifugal separation characteristics are explained. The viscosity of blood is related to that for suspensions of rigid particles and an equation for the hydraulic permeability of red blood cell suspensions is derived.