6-rheology of blood -...

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6-Rheology of blood

Non-newtonian fluid

(Biofluid mechanics, the human circulation chap 4)

6.1 Vicosymetry- Capilary viscosimeter- Couette viscosimeter- Faling sphere

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6.2 Blood composition

Blood cells 45%Red blood cellWhith blood cell (leukocytes )Platlet

Plasma 55%

Hematocrit (Ht or HCT) is the proportion of blood volume that is occupied by red blood cells45%

Blood volume in humans is 4 – 6 L

Blood makes up 8 % of total body weight

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Red blood cells

7 µm

2 µm

Also called erythrocytes

Properties:

Aggregation

Deformation

5 µm

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•Physiological and reversible phenomena

•Increase the blood viscosity

•Hyper-aggregation is associated with the cardiovascular disease pathologies

Red blood cell aggregation

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Time

Porcine blood

Human Blood

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6.2 Blood Viscosity

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Factors Affecting Blood Viscosity

• Shear rate– At low shear rates, apparent

viscosity increases. – Yield stress required to start

fluid motion (Bingham Fluid)

• Plasma composition

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Factors Affecting Blood Viscosity

• Cell concentration– Plasma is generally

considered to be a Newtonian fluid at physiological conditions ( = 1.6×10-3 kg/m·s)

– Increasing RBC concentration increases viscosity linearly (over a physiological range)

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Factors Affecting Blood Viscosity

• Cell deformation

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Factors Affecting Blood Viscosity

• Fibriogen concentration and hematocrit– Affects the yield

shear stress

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Factors Affecting Blood Viscosity

• Vessel diameter– Apparent viscosity is lower

in small vessels (d < 0.3 mm)

– Apparent viscosity approx. constant in vessels above 0.3 mm

• 6.3 Cell free marginal layer (Biofluid mechanics, the human circulation chap 4)

• 6.4 Pressure-flow relationship for non-newtonian fluid(Biofluid mechanics, the human circulation chap 4)

– power law– Bigham– Casson

• 6.5 Hemolysis and platetet activation• 6.6 Time effect of viscosity

6.5 Hemolysis and platetet activation

1. Damage to the endothelial layer of the blood vessel wall

2. Platelets begin to adhere to damage site

3. Platelets secrete chemicals which cause other platelets to adhere to the original ones

4. Clotting factors are released which convert fibrinogen to fibrin

5. Fibrin fibres attach to clot

http://www.merck.com/mmhe/sec14/ch173/ch173a.html

Blood clot

Thrombus• A thrombus is a blood undesired clot that forms

in a vessel where there is no external bleeding (vascular disease)

Thrombi in medical devices• Many flow factors contribute to thrombosis in

medical devices– Thrombosis is most common in areas of flow

stagnation– Additional factors include (Wood et al., 2005):

• Flow separation• Flow recirculation• High shear stresses or very low shear stresses• Surface roughness, sharp edges, or narrow gaps

Hemolysis

• Hemolysis is the destruction or damage of red blood cells

• haemoglobin is release into the surrounding blood

Hemolysis in medical devices• Due to a combination of high shear stress

and exposure time

6.6 Time effect of viscosity

Chen 1994

Chen 1994

Blood

Bureau 1978