Lecture 4 - Blood viscosity Flashcards
Give mathematical definition of viscosity.
F/A = tau_yx = mu(-dVx/dy)
Unit - Pa s
What is shear stress?
tau_yx = mu * ydot_yx
Unit - Pa OR N/m^2
What is shear rate?
ydot_yx
Difference in velocity between fluid layers
OR
Velocity gradient in direction perpendicular to that of shear
force
Unit - 1/s
What is mu?
Fluid Viscosity
Shear stress / Shear rate
What is a Newtonian fluid
shear stress/ shear rate (viscosity) is linear
What is dynamic viscosity? (eta)
nu = eta/rho
What does viscosity depend on in a newtonian fluid?
temperature
material
What makes up particle volume of blood?
99% RBC’s
What does blood viscosity depend on?
plasma viscosity and haematocrit
RBC size, shape, deformability, disease
Effect of higher hematocrit and less deformable
cells?
Higher viscosity
Einstein’s Formula? When is it valid?
eta = eta_plasma(1+1.25Ht)
only valid for low particle
concentrations
Name two non-newtonian blood models
power law
carreu model
Why is blood shear thinning?
At high shear rates RBCs orient in
direction of flow and viscosity is lower;
At low shear rates formation of RBC
aggregates occur, increasing viscosity to
high values
What is the Farhaeus-Lindqvist effect?
In small vessels (<1mm diam) and at low
velocities blood viscosity decreases with
decreasing vessel size
How is blood viscosity measured? Explain
viscometers, commonly cone viscometers
- two rotating surfaces (plates)
- Fluid is introduced into the space between the plates and the upper plate is spun at increasingly high speeds
3.Results are often device dependent - From the rotational speed the shear rate is calculated
