Viscosity of Biological Fluids

Viscosity of Biological Fluids 22.3.12

Viscosity is a property of flowing fluid which opposes relative motion of its layers in contact. Because of viscosity a force must be applied to move one layer of liquid past another • Types of fluids: Newtonian Non Newtonian Viscoelastic • A shear stress is defined as the component of stress parallel to the surface of the material • Any real fluid (liquids and gases included) moving along solid boundary will incur a shear stress on that boundary. The no-slip conditiondictates that the speed of the fluid at the boundary (relative to the boundary) is zero, but at some height from the boundary the flow speed must equal that of the fluid. The region between these two points is aptly named the boundary layer

Types of Fluid Flow • Laminar flow: In Laminar flow, the central axis of flow moves with the highest velocityand the successive cylindrical laminae moves progressively more slow as one moves from central axis to the wall Laminar flow has a parabolic front • Turbulent flow: A flow in which the elements of the fluid move irregularly in axial, radial and circumferential directions. It occurs at different velocities which fluctuates randomly. The velocity difference across the container changes erratically

Viscosity • For Newtonian fluids, viscosity is constant irrespective of flow rate. Viscous force (internal friction of layers of fluid) results due to relative motion between layers. Layers of flowing fluid move with different velocities due to shear stress • The viscous force is proportional to the surface area (A) of the layer and the velocity gradient between layers (in the direction perpendicular to the layer) Negative sign shows that F acts in a direction opposite to the one in which the layers move • η is the constant of proportionality and is the viscosity. It can be expressed as the ratio of shear stress and time rate of shear strain (or shear rate)

A spherical ball falling in a viscous fluid produces a shear stress • It reaches a terminal speed when the sum of the forces acting on it is zero • Laminar flow

For all Newtonian fluids in laminar flow the shear stress is proportional to the strain rate in the fluid where the viscosity is the constant of proportionality. • However for Non Newtonian fluids, this is no longer the case as for these fluids the viscosity is not constant • When flows are changing with time, such as blood flow in the human circulation, the liquid generally demonstrates both a viscous and an elastic effect, Such liquids are called viscoelastic • Blood plasma normally shows viscosity only while whole blood is both viscous and elastic

Viscosity of blood • Blood is a suspension of cells in plasma. Plasma is a water solution of salts and heavily hydrated macromolecules • The viscosity of blood thus depends on the viscosity of the plasma, in combination with the hematocrit (The hematocrit or packed cell volume (PCV) or erythrocyte volume fraction (EVF) is the volume percentage (%) of red blood cells in blood. It is normally about 45% for men and 40% for women) • Blood cells behave as suspended particles and increase the viscosity of blood. Fibrinogen due to its asymmetry and high density imparts maximum viscosity to the blood • Globulins have greater influence than albumin

Effect of Erythrocytes on viscosity of blood • Deformability of Erythrocytes • Haematocrit

Deformability of Erythrocytes • Erythrocyte deformability refers to the cellular properties of erythrocytes which determine the degree of shape change under a given level of applied force • Erythrocytes change their shape extensively under the influence of applied forces in fluid flow or while passing through microcirculation. • The extent and geometry of this shape change is determined by both the mechanical properties of erythrocytes, the magnitude of the applied forces and the orientation of erythrocytes with the applied forces

Deformability and viscosity • Erythrocyte deformability is an important determinant of blood viscosity, hence blood flow resistance in the vascular system • Deformability of erythrocytes affects viscosity of blood in small vessels where erythrocytes are forced to pass through blood vessels with diameters smaller than their size • In sickle cell anaemia, deformability of erythrocytes is reduced thereby increasing viscosity of blood

Elastic Erythrocytes • Erythrocyte when freely suspended assumes biconcave discoid shape indicative of large excess of its surface area over its volume • Shape change of erythrocytes under applied forces (i.e., shear forces in blood flow) is reversible and the biconcave-discoid shape is maintained after the removal of the deforming forces • In other words, erythrocytes behave like elastic bodies, while they also resist to shape change under deforming forces

The viscoelasticity of blood is traceable to the elastic red blood cells, which occupy about half the volume. • When the red cells are at rest they tend to aggregate and stack together • In order for blood to flow freely, the size of these aggregates must be reduced, which in turn provides some freedom of internal motion. The forces that disaggregate the cells also produce elastic deformation and orientation of the cells

In Region 1, the cells are in large aggregates • In Region 2, the cells are disaggregated and the applied forces are forcing the cells to orient. As the shear rate increases, the applied forces deform the cells • In Region 3, increasing stress deforms the cells, and if the cells have normal deformability they will form layers that slide on layers of plasma.

Effect of haematocrit on Blood Viscosity • Viscosity of blood also varies with changing haematocrit • When the haematocrit rises, the friction between the successive layers of blood increases. Hence with increasing haematocrit the viscosity of the blood rises drastically

Effect of Temperature on the Viscosity of Blood • This is due to the presence of thermoproteins, which show physical change s at temperatures below or above 37 °C • Cryglobulin operates at temperatures below 37 °C forming reversible or irreversible precipitates, gels or crystals • Pyroglobulin gets precipitated at 56 °C • Bence-Jones proteins gets precipitated at 40 °C • At 0 °C, the viscosity of blood is increased up to three times. This reduces the circulation in the tissues exposed to cold

Effect of Inflammation and disease conditions on Viscosity of Blood • Inflammation causes loss of plasma into the tissues. This leads to slowing of blood flow, disturbance in the axial flow, rouleaux formation and increase in viscosity • Blood viscosity is increased in diabetes mellitus, multiple myloma, jaundice, leukaemia, asphyxia, vomiting and diarrhea Typical mammalian erythrocytes: (a) seen from surface; (b) in profile, forming rouleaux

Viscosity of Sinovial Fluid • Synovial fluid is highly viscous. Its viscosity varies from 50 to 200 times that of water. Its viscosity is due to the presence of hyaluronic acid • Viscosity of Synovial fluid reflects its hyaluronicacid content • In inflammatory effusions such as rheumatoid arthritis etc , the viscosity of synovial fluid is markedly decreased (down to water like) indicating the presence of a thin watery fluid containing degraded small molecular hyaluronidase particles

Viscosity of gels • Dietary fibre is a complex carbohydrate. Non-starch polysaccharides that are soluble in water form viscous solutions or gels • Viscous properties of this gel delays gastric emptying, reduce absorption from small intenstine, cause greater exposure of ileum to fat • The presence of fat in the ileum suppresses the motility of stomach, hence inducing satiety sensation earlier to a meal • Dietary fibremay affect gastric emptying in several ways. First, they may slow gastric filling, when certain soluble fibres are mixed in liquid meals or in liquid/solid meals, they delay emptying of gastric liquids by increasing viscosity of gastric contents. Such an increase in the viscosity of chyma could also slow the gastric emptying of solid components of the meal

Viscosity of Biological Fluids