Biomedical Engineering Reference
In-Depth Information
size as the red blood cells, ranging from 3 to 8 μm (Popel and Johnson 2005; Pries
et al. 1992). When blood flows through these vessels, red blood cells deform con-
siderably causing microcirculation and increasing viscosity as the vessel diameter
approaches the theoretical minimum blood cell diameter of 2.7 µm.
Finally like all fluids, (e.g. water) viscosity is dependent on temperature. A de-
crease of 1 °C in temperature yields approximately a 2 % increase in the viscosity
of blood. Thus in colder regions of the body such as our fingertips, the viscosity of
blood is higher than in that found in the brain. Furthermore in a case of hypothermia
where the temperature is very low, the blood viscosity increases impeding on blood
flow.
Representing blood as a non-Newtonian fluid can be achieved by using the blood
apparent viscosity power-law,
(4.2)
TT n
0
/
−
1
µ
=
ke
S
where
k
= 0.00622 [kg s
n−2
/m] is the consistency index,
n
= 0.7 is the powerlaw in-
dex,
T
[K]and
T
o
[K] are the local and reference temperatures, respectively, and
S
is the shear rate.
4.4
Clinical Relevance of Blood Viscosity
Haematocrit levels and its corresponding viscosity provide insight to the health im-
plications of a patient. Abnormally high haematocrit levels of 60-70 % are called
polycythaemia where there is increased oxygen content and increased viscosity.
This leads to an increased resistance to blood flow, straining the heart to work hard-
er and can impair organ perfusion. Similarly athletes that perform blood doping by
injecting packed red blood cells, or illicit products such as erythropoietin (EPO),
increase their haematocrit to increase oxygen delivery. This elevates blood viscos-
ity and increases the workload on the heart. Conversely low haematocrit < 39 %
leads to anaemia which produces decreased oxygen and decreased viscosity. Some
common causes include trauma leading to loss of blood, bone marrow disorders,
dehydration, lung disease and the abuse of certain drugs.
Increased viscosity has been linked with major cardiovascular risk factors in-
cluding high blood pressure, cholesterol, obesity, smoking, and age (Lowe et al.
1997; Sloop 1996). Blood vessels affected by these cardiovascular risk factors hin-
der natural blood flow. When blood flow slows down, viscosity increases as a result
of more cell-to-cell and protein-to-cell adhesions. Blood cells start to bind together,
forming clumps that thicken the blood, potentially clotting the vessels. Since vis-
cosity describes the thickness or stickiness of a fluid, it also describes the resistance
of blood to flow. This leads to a measure of how hard the heart has to work to deliv-
ery blood to the body, a key measure of cardiovascular performance.
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