Mechanical trauma to flowing blood exposed to non-physiological forces has been studied extensively in vitro and in vivo for nearly 50 years. The concept of critical thresholds of shear stress lytic to RBCs was introduced in works of Nevaril et al. in 1968, Leverett et al. in 1972 and Williams in 1972. Historically, the most commonly studied aspect of blood trauma was shear-induced hemolysis which still remains a clinically relevant complication of assisted circulation. Early experiments established an inverse relationship between critical shear stress and time exposure; meaning that blood is able to sustain greater shear for shorter periods of time before hemolysis occurs. The so called “Leverett curve” (Figure 3-1) represents collections of published and unpublished results on the shear stress and exposure time combinations in experiments which produced significant hemolysis. This curve was suggested to be used to segregate hemolysis data which originated from blood exposure to extremely high shear stresses for short periods (on the right side of the line), from a prolonged exposure of blood to foreign surfaces at lower shear stresses (on the left side of the line).