Biomedical Engineering Reference
In-Depth Information
Fig. 4.1
Blood is a liquid tissue made up of blood cells and platelets
(e.g. change in velocity) are known as a Newtonian fluid. Blood is a heterogeneous
fluid containing blood cells that change in shape and movement within the plasma
during velocity changes. This leads to a variable viscosity making blood a non-
Newtonian fluid (this is discussed in more detail in the next section). In many hae-
modynamic studies, blood viscosity is assumed constant (typically 3.2 × 10
−3
Pa.s)
because only small changes in the blood are found throughout a given study and this
change is negligible.
The viscosity of a fluid is quantified by the dynamic and kinematic viscosity. The
dynamic viscosity,
µ
and has standard units of Newton seconds per square metre,
2
kgm
−−
The kinematic viscos-
ity, ν, is the ratio of dynamic viscosity to the fluid density
Nsm
−
(
1)
(
1)
or kilograms per meter per second
[
]
[
].
νµρ
=
/
which has units
ms
−
The
density
of blood is approximately 1060 kg/m
3
at 37 °C and is related the vol-
ume of red blood cells in blood. Only to a minor extent is the density influenced by
other plasma solutes (Kenner 1989). Comparatively the density of water is 998 kg/m
3
at 20 °C and the density of air is 1.22 kg/m
3
at standard atmospheric pressure and
temperature.
In general the density of a fluid changes with temperature and pressure, but
this is typically small for liquids and therefore is often assumed constant de-
spite any variation in temperature and pressure. To exemplify this, we con-
sider a fluid's compressibility which is the fractional change in volume (m
3
) per
unit increase in pressure or temperature. A volume change will alter its density
(since its mass remains the same); however for a liquid this is typically involves
a change of 0.001 m
3
per 1 kPa of pressure change and 0.00001 m
3
/1 °C change
in temperature. Such small volume changes have a small effect on its overall
density.
2
(
1)
metres squared per second
[
].
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