Summary. The microrheology and bulk viscosity of whole blood, erythrocytes and crythrocyte aggregates was examined at temperatures between 37 and 4°C. Beside the well established increases of blood and plasma viscosity at high rates of shear, a specific increase of blood (not plasma) viscosity at low rates of shear is seen below 20°G. This increase is not seen in non-aggregating blood samples (red cells in isotonic albumin solution). At the same temperature and shear rate range, a strong increase in the tendency to aggregation is regularly observed. The shear stresses necessary to keep red cell aggregates dispersed in flow increase from 3 (at 37 °) to 10 dynes/cm ~ (at 4°C). The deformability of the cells as tested by their ability to pass 5 ~m pores is, however, not abolished by lowering temperatures. These in vitro results give strong support to the notion that the microeirculatory disturbances seen in response to hypothermia in vivo are indeed related in part to increased tendency to red cell aggregation.Key words: Aggregometry --Apparent blood viscosity --Erythrocyte aggregation --Erythrocyte deformability --Hemoglobin viscosity --Hypothermia --Plasma viscosity.Zusammenfassung. Mikrorheologie und ViscositEt yon Vollblut, Erythrocyten und Erythroeytenaggregaten wurden bei Temperaturen zwischen 37 und 4 ° C untersueht. Neben lange bckannten Viscosit~,tssteigerungen yon Blur und 1)lasma bei hohen Schergraden wurde ein spezieller Anstieg der Blutviscositiit (nieht Plasmaviscositiit) bei niedrigen Schergraden und Temperaturen unter 20°C beobachtct. Dieser Anstieg fehlt in nichtaggregierenden Blutproben (ErythroeyCen in isotonischcr AlbuminlSsung). Im gleichen Tempcratur-nnd Schergradbereich wurde rcgelmEBig eine starke Zunahme der Aggregationstendenz beobachtet. Die Sehubspannungen, die zur Dispersion von Aggregaten notwendig sind, steigen yon 3 dyn/cm 2 bei 37°C auf 10 dyn/cm ~ bei 4°C. Die Verformbarkeit der Zellen, getestet an ihrer FEhigkeit, dutch 5 ~zm Porch zu flieBen, ist jedoeh nicht aufgchoben. Diese in vitroErgebnisse untcrstfitzen die Vorstellung, dab die MikrozirkulationsstSrungen in vivo nach Hypothermie tats~chlich auf gesteigerter Aggregationstendenz der Erythrocyten bcruht.The microrheology of blood, i.e. the detailed study of the flow properties of blood cellular elements, has added m u c h to our knowledge of the factors responsible for both the blood flow in the living mierocireulation and for the soealled anomalous viscosity of blood in vitro. Phenomenologi-
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