Hemolysis in clinical blood samples leads to inaccurate assay results and often to the need for repeated blood draws. In vitro experiments were conducted to determine the influence on hemolysis in phlebotomy needles and catheters of pressure difference, cannula diameter, and cannula material. Fresh blood from five human volunteers was forced from a syringe inside a pressurized chamber through 14, 18, and 22 gauge 304 stainless steel needles and polyurethane and Teflon catheters, all 40 mm long. Hemolysis was measured in the samples by a spectrophotometer. It was found that hemolysis increased with increases in pressure difference and cannula diameter and no consistent trend could be identified with regard to cannula material. The pressure differences required for significant hemolysis were above those typical of clinical venipuncture blood draws. While there was substantial variability among individuals, the hemolysis values scaled with exponent S = (t/t0)[(tau/tau0)-1]2, where t is the characteristic duration of shear, t0 is a time constant, tau is the wall shear stress, and tau0 is the wall shear stress threshold below which no hemolysis occurs. A hemolysis threshold including both time and shear stress was also defined for S = constant. The threshold implies that a threshold shear stress exists below which erythrocytes are not damaged for any length of exposure time, but that red cells may be damaged by an arbitrarily short period of exposure to sufficiently large shear stress.

The rate of spread of a passive species is modified by the superposition of a velocity gradient on the concentration field. Taylor (18) solved for the rate of axial dispersion in fully developed steady Newtonian flow in a straight pipe under the conditions that the dispersion be relatively steady and that longitudinal transport be controlled by convection rather than diffusion. He found that the resulting effective axial diffusivity was proportional to the square of the Peclet number Pec and inversely proportional to the molecular diffusivity. This article shows that under similar conditions in Casson and power law fluids, both simplified models for blood, and in Bingham fluids the same proportionalities are found. Solutions are presented for fully developed steady flow in a straight tube and between flat plates. The proportionality factor, however, is dependent upon the specific rheology of the fluid. For Bingham and Casson fluids, the controlling parameter is the radius of the constant-velocity core in which the shear stress does not exceed the yield stress of the fluid. For a core radius of one-tenth the radius of the tube, the effective axial diffusivity in Casson fluids is reduced to approximately 0.78 times that in a Newtonian fluid at the same flow. Using average flow conditions, it is found that the core radius/tube radius ratio is 0 (10(-2)) to 0 (10(-1)) in canine arteries and veins. Even at these small values, the effective diffusivity is diminished by 5% to 18%. For power law fluids, Pec2 dependence is again found, but with a proportionality constant dependent upon the power law exponent n.(ABSTRACT TRUNCATED AT 250 WORDS)

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