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S17.6. Models for slow blood flow in narrow tubes; Effects of aggregation and sedimentation of flow resistance
Vol. 32, Nos. 2-3
Symposia
169
S17.6. MODELS FOR SLOW BLOOD FLOW IN NARROW TUBES: EFFECTS OF AGGREGATION AND S E D I M E N T A T I O N O F FLOW RESISTANCE T. W. SECOMB AND A. W. EL-KAREH Department of Physiology, University of Arizona, Tucson, AZ 85724, USA The apparent viscosity of blood in microvessels strongly influences flow resistance in the microcirculation. At low blood shear rates, as occur in venules, red blood cells can aggregate, resulting in relatively wide cell-free or cell-depleted regions at the vessel wall, surrounding a high-hematocrit core. If this core remains near the center-line, as in vertical tubes, apparent viscosity is reduced. However, the core is denser than the surrounding fluid, and sedimentation can occur in non-vertical tubes, increasing the apparent viscosity. Here, we describe theoretical analyses of these phenomena. The transition from dispersed to fully aggregated flow in vertical tubes is represented by a model in which the size of the aggregated core is determined by the balance between the processes of attachment and detachment of red cells. For a fully aggregated core, the time-scale of sedimentation, and its effects on flow resistance, are modeled by considering the motion of a rigid eccentric cylinder within a fluid-filled tube. The predicted time-scale for sedimentation is about 60 s for a horizontal tube with diameter 100 Bm, consistent with observations. Apparent viscosity is shown to increase rapidly with increasing size and eccentricity of the core. In the presence of sedimentation, a gradual build-up of hematocrit within the vessel is predicted, These p h e n o m e n a may influence the resistance to blood flow in venules. (Supported by NIH grants HL34555 and HL07249).
S17.7.
THE
AGGREGATION RHEOLOGICAL
DISTURBANCE BY ERYTHROCYTE OF BLOOD FLOW STRUCTURING AND PROPERTIFS
IN LIVING MICROVESSFJ.S
G. MCHEDLISHVILI AND L. GOBEJISHVILI Microcirculation Research Center, I. Beritashvili Institute of Physiology, !4 Gotua Str., 380060 Tbilisi, Republic of Georgia Proceeding from perennial analysis of blood flow in living microvessels, the structuring of erythrocyte flow was inferred to be the major determinant of blood rheological properties in microcirculation. As to the factors disturbing these properties, it was found to be primarily the intensified erythrocyte aggregation, which deranges all elements of the structure. It disintegrates red cell axial flow, their velocity profile, uniformity of local hematocrit along microvessels, etc. In addition, positive feed-backs originate in microvascular beds: rise in resistance caused by the aggregation, results in slowing-down of flow and enhances, in turn, the aggregation, thus leading to development of blood stasis. Furthermore, erythrocyte aggregation is a feature of blood flow structuring in microvessels, which has no, or almost no, positive significance for microcirculation, as processes, e.g. high hematocrit, increased fibrinogen content in blood plasma, etc. Techniques for investigating erythrocyte aggregability are of paramount significance from the clinical point of view.
Symposia
169
S17.6. MODELS FOR SLOW BLOOD FLOW IN NARROW TUBES: EFFECTS OF AGGREGATION AND S E D I M E N T A T I O N O F FLOW RESISTANCE T. W. SECOMB AND A. W. EL-KAREH Department of Physiology, University of Arizona, Tucson, AZ 85724, USA The apparent viscosity of blood in microvessels strongly influences flow resistance in the microcirculation. At low blood shear rates, as occur in venules, red blood cells can aggregate, resulting in relatively wide cell-free or cell-depleted regions at the vessel wall, surrounding a high-hematocrit core. If this core remains near the center-line, as in vertical tubes, apparent viscosity is reduced. However, the core is denser than the surrounding fluid, and sedimentation can occur in non-vertical tubes, increasing the apparent viscosity. Here, we describe theoretical analyses of these phenomena. The transition from dispersed to fully aggregated flow in vertical tubes is represented by a model in which the size of the aggregated core is determined by the balance between the processes of attachment and detachment of red cells. For a fully aggregated core, the time-scale of sedimentation, and its effects on flow resistance, are modeled by considering the motion of a rigid eccentric cylinder within a fluid-filled tube. The predicted time-scale for sedimentation is about 60 s for a horizontal tube with diameter 100 Bm, consistent with observations. Apparent viscosity is shown to increase rapidly with increasing size and eccentricity of the core. In the presence of sedimentation, a gradual build-up of hematocrit within the vessel is predicted, These p h e n o m e n a may influence the resistance to blood flow in venules. (Supported by NIH grants HL34555 and HL07249).
S17.7.
THE
AGGREGATION RHEOLOGICAL
DISTURBANCE BY ERYTHROCYTE OF BLOOD FLOW STRUCTURING AND PROPERTIFS
IN LIVING MICROVESSFJ.S
G. MCHEDLISHVILI AND L. GOBEJISHVILI Microcirculation Research Center, I. Beritashvili Institute of Physiology, !4 Gotua Str., 380060 Tbilisi, Republic of Georgia Proceeding from perennial analysis of blood flow in living microvessels, the structuring of erythrocyte flow was inferred to be the major determinant of blood rheological properties in microcirculation. As to the factors disturbing these properties, it was found to be primarily the intensified erythrocyte aggregation, which deranges all elements of the structure. It disintegrates red cell axial flow, their velocity profile, uniformity of local hematocrit along microvessels, etc. In addition, positive feed-backs originate in microvascular beds: rise in resistance caused by the aggregation, results in slowing-down of flow and enhances, in turn, the aggregation, thus leading to development of blood stasis. Furthermore, erythrocyte aggregation is a feature of blood flow structuring in microvessels, which has no, or almost no, positive significance for microcirculation, as processes, e.g. high hematocrit, increased fibrinogen content in blood plasma, etc. Techniques for investigating erythrocyte aggregability are of paramount significance from the clinical point of view.