Blood rheology.

Abstract

Rheology, the science of the deformation and flow of matter, has become of considerable interest to haematologists, and now the measurement of blood and plasma viscosity is a familiar part of the investigation of vascular disorders and the paraproteinaemias. Developments in instrumentation have led to the measurement of plasma viscosity and the zeta sedimentation ratio as practical alternatives to the erythrocyte sedimentation rate, and a variety of methods have also been introduced for the study of red cell deformability. It is therefore an appropriate time to review these recent developments and to examine their potential application to the investigation and management of clinical disorders. Determinants of viscosityAll fluids resist, to a greater or lesser extent, attempts to alter their shape, and this resistance to flow is a measure of a fluid's viscosity. During flow, as layers of fluid move parallel to one another at different rates, a velocity gradient forms between these layers and is known as the shear rate; it is measured in reciprocal seconds (s-1). The force required to produce this velocity gradient is the shear stress and is measured in newtons per square metre (NM-2). Viscosity can now be redefined as the ratio of shear stress to shear rate, the unit of viscosity being the pascal second (Pa s; conversion factor -1 mPa s = 1 centipoise).Simple fluids, such as plasma and most oils, show a linear relationship between shear stress and shear rate (Newtonian behaviour) so that the viscosity remains constant. Whole blood, however, behaves as a non-Newtonian fluid in that viscosity increases exponentially at the low shear rates (below 50 s-1) that characterize venous flow. This increase is due to the larger molecular weight plasma proteins (fibrinogen and certain globulins) which overcome the zeta potential between erythrocytes and form rouleaux; these large cellular aggregates cause a disproportionate increase in viscosity. At shear rates below 1 s51, alterations in plasma fibrinogen around the upper limit ofthe physiological range have a pronounced

Patient-specific blood rheology in sickle-cell anaemia

“…about 18.6-21.9% compared to about 40-45% normally). It is known that haematocrit is the most obvious determinant of blood viscosity [37]. A decrease in haematocrit leads to a decrease in blood viscosity.…”

Hematocrit, blood pressure, and hypertension. The Gubbio Population Study.

“…These estimates are close to the blood pressure changes observed in anemic patients after hematocrit is increased by blood transfusion or by erythropoietin treatment. 14 -20 Reasonable mechanisms underlying the association between hematocrit and blood pressure are the relations of hematocrit with whole blood viscosity, 1 of whole blood viscosity with peripheral resistance, and of peripheral resistance with blood pressure. 2 It has been proposed that with treatment of anemia, cessation of vasodilation (previously present because of tissue hypoxia) and increased blood viscosity, both resulting from the therapeutic increase in hematocrit, account for the increased peripheral resistance with rise of blood pressure observed.…”

“…1 Blood viscosity and vascular resistance affect total peripheral resistance to blood flow, 2 which is abnormally high in the established phase of primary hypertension. 3 It has been reported that hematocrit is increased or normal in human hypertension*- 9 and increased in rat genetic hypertension.…”

Erythrocyte rheological properties but not whole blood and plasma viscosity are associated with severity of hypertension in older people

“…Blood viscosity and vascular resistance affect total peripheral resistance to blood flow [1], which is abnormally high in the established phase of primary hypertension [2]. Blood flow, deformability and aggregation of erythrocytes are the main components of hemorheology.…”