Blood viscoelasticity

Thurston, G.B. (1979) Erthrocyte rigidity as a factor in blood rheology viscoelastic dilatancy, /. KheoL, 23, 703-19 (1979) Blood viscoelasticity and relaxation processes influence of aggregation tendency. Symposium European, Hemotheologie et Pathologie, Nancy, France, 17-19 Oct (1979)... 

Detailed treatments of the rheology of various dispersed systems are available (71—73), as are reviews of the viscous and elastic behavior of dispersions (74,75), of the flow properties of concentrated suspensions (75—82), and of viscoelastic properties (83—85). References are also available that deal with blood red ceU suspensions (69,70,86). 

The human erythrocyte possesses a characteristic biconcave shape and remarkable viscoelastic properties. Electron microscopy studies performed on red blood cells (RBC), ghosts, and skeletons revealed a two-dimensional lattice of cytoskeletal proteins. This meshwork of proteins was thought to determine the elastic properties of the RBC. This... 

Fibrin is a viscoelastic polymer, which means that it has both elastic and viscous properties (Ferry, 1988). Thus, the properties of fibrin may be characterized by stiffness or storage modulus (representing its elastic properties) and creep compliance or loss modulus/loss tangent (representing its inelastic properties). These parameters will determine how the clot responds to the forces applied to it in flowing blood. For example, a stiff clot will not deform as much as a less stiff one with applied stress. 

Humans and other organisms are filled with complex fluids. Blood is a suspension containing around 40% by volume blood cells, which are flexible puckered disks roughly 10 / m in diameter, suspended in clear plasma, which is itself a viscoelastic fluid containing interacting macromolecules. The viscoelastic properties of blood determine the pumping load... 

Thurston GB. Rheological parameters for the viscosity, viscoelasticity and thixotropy of blood. Biorheology 1979 16 149-162. 

To provide a complete assessment of all these variables, the final evaluation of the safety of ophthalmic products must be made in the in vivo model using the preparation under the proposed conditions for use, following tissue compatibility with many of the techniques already discussed. Irrigating solutions of low viscosity may have limited contact, while the gel-like viscoelastic materials, which maintain the comeal dome, or the solutions and gases used as vitreous replacements to prevent retinal detachment, may have prolonged contact with delicate ocular tissues or the retina. A recent therapy involves treatment for neovascularization of the retina, a disease in which proliferation of blood vessels can lead to blindness. The treatment combines a systemic chemical that localizes... 

The arterial circulation is a multiply branched network of compliant tubes. The geometry of the network is complex, and the vessels exhibit nonlinear viscoelastic behavior. Flow is pulsatile, and the blood flowing through the network is a suspension of red blood cells and other particles in plasma which exhibits complex non-Newtonian properties. Whereas the development of an exact biomechanical description of arterial hemodynamics is a formidable task, surprisingly useful results can be obtained with greatly simplified models. 

The viscoelasticity of blood vessels affects the hemodynamics of arterial flow. The primary function of arterial elasticity is to store blood during systole so that forward flow continues when the aortic valve is closed. Elasticity also causes a finite wave propagation velocity, which is given approximately by the Moens-Korteweg relationship... 

Vasodilator A substance which causes a decrease in smooth-muscle tone, thereby dilating blood vessels. Viscoelastic A substance which exhibits both elastic (soHd) and viscous (Hquid) characteristics. 

Constitutive equations that capture the essential features of the responses of red blood cells and passive leukocytes have been formulated, and material parameters characterizing the ceUular behavior have been measured. The red ceU response is dominated by the ceU membrane which can be described as a hyper-viscoelastic, two-dimensional continuum. The passive white ceU behaves like a highly viscous fluid drop, and its response to external forces is dominated by the large viscosity of the cytosol. Refinements of these constitutive models and extension of mechanical analysis to activated white ceUs is anticipated as the ultrastmctural events that occur during ceUular deformation are delineated in increasing detail. 

There are several major techniques that are used to extract the mechanical properties of ceUs. The models and experiments are interconnected the experiments provide parameters for the models and, in turn, the models are the basis for the interpretation of the experiments. One common technique is micropipette aspiration, where a pipette is sealed on the surface of a cell, negative pressure is appUed inside the pipette, and a portion of the ceU is aspirated into the pipette. The height of the aspirated portion is considered as an inverse measure of the ceU stiffness. The same technique is used to observe the time response of the ceU to the appUcation of pressure, and in this case, the corresponding relaxation time is a measure of the cell s viscoelastic properties. The experiment with the micropipette aspiration of a red blood ceU was interpreted by considering the ceU membrane (including the cytoskeleton) as a nonlinear elastic half-space... 

Most fluids exhibit non-Newtonian behavior—blood, household products like toothpaste, mayonnaise, ketchup, paint, and molten polymers. As shown in Figure 7.9, shear stress, t, increases linearly with strain rate, y, for Newtonian fluids. Non-Newtonian fluids may be classified into those that are time dependent or time independent and include viscoelastic fluids. Shear thinning (pseudoplastic) and shear thickening (dilatant) fluids are time independent while rheopectic and thixotropic are time dependent. The shear stress (viscosity) of shear thinning fluids decreases with increasing shear rate and examples include blood and syrup. The viscosity of dilatant fluids increases with shear rate. The viscosity of rheopectic fluids—whipping cream, egg whites—increases with time while thixotropic fluids— paints (other than latex) and drilling muds— decrease their viscosity with the duration of the shear. 

---