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L3 Blood Flow and Blood Rheology
Factors controlling blood viscosity
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If it sediments down far, have lots of aggregation due to high
fibrinogen levels = sick
Blood clotting
 Viscosity of blood increases in proportion to viscosity of the plasma
 Lesser factors
o Red cell aggregation
 Increases viscosity at low shear (speed) rate
 Mainly induced by fibrinogen
o Red cell deformation (elongation) higher speed
 Causes viscosity, but decrease as shear rate increases
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In the middle the layers are mostly going the same speed
o Shear rate varies between different regions of circulation therefore local viscosity varies
o E
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arterioles highest shear rate, venules and veins lowest

Blood Viscosity in Intermediate Sized Vessels
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Diameter <100μm (a lot of arterioles – where a lot the of circulation resistance comes from)
Viscosity is lower in small vessels than in large vessels
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o Therefore, have a lower resistance that you think
Plug flow
o Not a lot of shear (form a few aggregates) so they are all going down at the same speed
o High shear near the wall
o Pushes platelets and white cells towards the outsides – helpful cos that’s were their function is

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The ‘apparent viscosity’ decreases with decreases vessel diameter
Haematocrit in many arterioles is lower than the systemic arterioles
o Flow resistance is lower than expected from systemic viscosity
o However, resistance still depends on haematocrit

White cells and platelets have negligible effects on blood viscosity
Clinical/ Physiological Indices
BLOOD: done routinely
 Haematocrit
o Oxygen carrying capacity
o Blood viscosity
 Erythrocyte sedimentation rate (ESR)
PLASMA: only in research
 Plasma viscosity
o Affects blood viscosity
 Fibrinogen concentration
RBCs:
 Blood content
o Haematocrit
o Red Cell count
o HB concentration in blood
 Cellular Characteristics
o Mean cell volume (MCV of red cell)
o Mean cell haemoglobin (MCH)
o Mean cell haemoglobin concentration (MCHC)
Consequences of Anaemia (low viscosity)
o Reduced O2 carrying capacity
o Oxygen saturation likely to be normal
o Reduced viscosity and resistance to flow
o Cardiac output may be high
Blood Rheology and Circulatory Pathology
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Elevated immunoglobulins
b
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Hyperleukotic leukaemia
d
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g
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Acute Phase Response (chronic) – response to damage or trauma
a
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Vascular disease
c
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Smoking
e

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