Biorheology

There have been a number of investigations on coagulation reactions of blood as well as on coagulation factors including fibrinogen and thrombin. With the progress of clotting, the viscosity and rigidity of blood increase, facts related to the clot structure of fibrin. Therefore, rheological measurements make it possible to investigate the process of blood clotting as well as the properties of the fibrin clot. In, this paper, our rheological studies on blood coagulation are summarized. The network structure of fibrin clots formed under different conditions is discussed from the kinetic analysis of the change of dynamic rigidity modulus during clotting. Also it will be shown that rheological techniques make it possible to analyze the initial coagulation reaction of blood in contact with components constituting the vascular vessel. The analysis indicates that a coagulation factor in plasma is activated on the erythrocyte surface.

Using a large scale model and the concept of dimensional similarity, we have developed a dimensionless correlation which permits the estimation of the drag force of blood flow in an in vivo vessel on a leukocyte adherent to the vessel wall. This relationship is Re · C d = 8.15 − 7.52 ln [ d / D ] 2 where Re is a Reynolds number, C_d is a drag coefficient, and d/D is the leukocyte-to-vessel diameter ratio. This function increases rapidly with increasing d/D when d/D is greater than 0.5. A relationship between the drag force on the leukocyte, F_f, and the commonly used wall shear stress in the absence of the adherent cell, τ w , has also been developed, and is given by F f τ w D 2 = 0.598 + 1.847 ln [ d / D ] 2 This study shows that the discrepancy between the in vivo and in vitro critical wall stresses (above which leukocytes do not adhere to endothelial cell layers) is not due to misrepresentation of the corresponding drag forces on the leukocytes. The discrepancy must be due to real differences in the flow and/or cellular conditions between the in vivo and in vitro experiments.

A method based on dielectric properties of cellular suspensions was developed to study red blood cell (RBC) aggregability. The time-dependent current in a Couette-type viscometer was recorded after abrupt stoppage of shearing. Since the current reaches steady state 2 min after the end of shearing, the observed effects were quantified by the relative current difference, Δ I rel = ( I 2 min − I 5 s ) / I 2 min , where subscripts designate the time of measurements. Δ I rel increases with hematocrit, plasma and fibrinogen concentration. The dependence of Δ I rel and of RBC aggregability on the concentration of dextran were similar. The experimental data and their analysis indicate that in suspensions with aggregating media, the Δ I rel value measured in the field of the β-dispersion reflects the difference between the size of aggregates under steady-state conditions and that of dispersed particles 5 s after the end of shearing. Therefore, this value may serve as a measure of RBC aggregability.

Much attention has been paid to the study of blood flow in long, narrow tubes. While the influence of tube diameter and driving pressure have been examined in detail, the influence of suspending phase viscosity has generally been assumed only to affect the blood viscosity in a linearly proportional manner, hence the practice of normalizing apparent blood viscosity values by the suspending phase viscosity to give a relative viscosity (e.g., Pries et al., 1992). While this assumption is probably valid for long tubes, it apparently does not hold for blood flow in short tubes (and by extension also for flow in short or branching capillary segments in vivo) in which RBC deformation plays a more significant role. In this paper we present a series of experiments using the Cell Transit Analyzer (CTA) in which the influence of driving pressure and suspending phase viscosity on RBC passage through short, narrow tubes has been systematically evaluated. Over the range studied (1 to 10 cm water), the influence of driving pressure was found to be unremarkable, in that RBC velocity scaled directly and linearly with pressure. This finding is consistent with previous studies. However, a distinct intercept was observed in the linear relationship between RBC pore transit time and suspending phase viscosity, which presumably arises as a consequence of RBC deformation either at the pore entrance or within the pore. Two simple mathematical models for the suspending phase-viscosity/transit-time relationship were considered. The results show that making CTA measurements over a range of suspending medium viscosities is a simple and practical way to obtain additional information about RBC mechanical properties.

The Cell Transit Analyzer (CTA) is now being used widely in clinical hemorheology. Most of the data obtained by CTA are limited to human blood, although the CTA has an important potential to be used in experimental studies on animal models. However, behavior of red blood cells (RBC) from various species might be different in CTA. Eight parameters reflecting different aspects of cell passage through pores with 5 µm diameter and 15 µm length were determined or human, guinea pig, dog, rabbit, rat, mouse and sheep RBC, together with instrument precision and biological variation. These parameters have a wide range when measured in different species and correlate with cell volume. Sensitivity of these parameters to the glutaraldehyde-induced alterations in RBC deformability was not same for different laboratory mammals. The main reason for this difference seems to be related to the cell size and thus sensitivity might be significantly limited if 5 µm pore-size filters are used to test the smaller RBC. The results of this study may help in designing experimental studies on laboratory mammals using the CTA.