To quantify the effects of major surface structural factors influencing interfacial reactions induced by polymers in native blood, model surfaces of solvent-cast films of two analogous poly (ether urethanes) and three homologous polyamides (nylon 4, 6/6, and 12) were exposed ex vivo to canine blood under the well-defined hemodynamic conditions of the Stagnation Point Flow Experiment. The selected surfaces allow for incremental changes in properties and were characterized by their “Composite Surface Free Energy Function,” γ′s, which describes the surface force field as the sum of the mean dispersion (equation imagesd) and polar (equation imagesp) contributions and is computed from wettability spectra obtained with ultrapure diagnostic liquids. Blood interfacial effects were measured by the shear-limited diameter of the white cell circle formed around the stagnation point, the flow parameter at which symmetric aggregation occurred, and the surface-number density of platelets, [Ps], remaining adherent under fixed conditions. At identical flows, within each group of polymers, both the WBC-circle diameter and [Ps] scale with equation imagesp/γ′s, implying that (1) only the magnitude but not the interaction mechanism varies as a function of incremental structural and surface changes, (2) the primary determinant of surface-induced effects is the polar force contribution, and (3) the magnitude of γ′s is secondary if equation imagesd/γ′s is sufficiently great.