Biomedical Engineering Reference
In-Depth Information
tissue. The arterial side of the vascular system is under high pressure. The arteries have a
mean pressure in the range of 90 mmHg to 100 mmHg, whereas the arterioles have a mean
pressure that ranges from 40 mmHg to 80 mmHg. Arterioles are the major resistance ves-
sels in the circulatory system.
5.2.
Veins are blood vessels that transport blood towards the heart and the venous wall is also
composed of three layers. The major difference between the arterial wall and the venous
wall is the thickness of the tunica media; the arterial wall is typically thicker than the
venous wall. Another major difference between the arterial anatomy and the venous anat-
omy is that veins contain valves to prevent the backflow of blood toward the capillary
beds. Flow through the veins is carried out under low pressure and is facilitated through
the contraction of skeletal muscles neighboring the veins termed the venous pump.
5.3.
Blood is composed of two major components, the cellular component and the plasma com-
ponent. Under normal conditions, the cellular component accounts for approximately 40%
of blood. The cellular component is divided into the red blood cells (greater than 99% of all
cells), the white blood cells (approximately 0.2%), and the platelets (approximately 0.1%).
Red blood cells are primarily responsible for the delivery of oxygen and carbon dioxide.
White blood cells are primarily responsible for protection against foreign particles (e.g.,
inflammatory responses). Platelets are responsible for hemostasis. The plasma component
consists of water, proteins, and other solutes such as sugars and ions, among others.
5.4.
Rheology is the study of a flowing material combined with the study of the physical prop-
erties of that material. Plasma behaves as a Newtonian fluid and has a viscosity of approxi-
mately 1.2 cP. Whole blood, however, does not have a constant viscosity. The viscosity of
whole blood varies with shear rate, hematocrit, temperature, and disease conditions, and
this is predominantly due to the presence of cells and other compounds within the fluid.
The Casson model for blood states
p
ηγ
p
5
p
τ
y
:
1
τ
y
is a constant yield stress
as a relationship between shear stress and shear rate, where
(which varies based on hematocrit) and
is an experimentally fit constant, which approxi-
mates the fluids viscosity. Under most physiological conditions, the yield stress of blood
can be approximated as 0.05 dyne/cm
2
. The velocity profile of a fluid modeled with the
Casson flow can be described by
η
<
:
0
@
1
A
1
4
dp
dx
8
3
r
0
:
5
R
2
r
2
R
1
:
5
r
1
:
5
ð
Þ
1
2
r
y
ð
R
r
Þ
r
y
#
r
R
2
2
2
2
2
#
y
η
0
@
1
A
u
ð
r
Þ
5
3
p
p
R
1
4
dp
dx
p
r
y
1
3
p
r
y
r
r
y
2
2
1
#
η
Red blood cells can form aggregates termed rouleaux. Interestingly, the presence of rou-
leaux in the blood is dependent on the shear rate, at which the fluid is flowing. At lower
shear rates, rouleaux are more prevalent, likely due to the greater time that cells can come
into contact with each other and/or proteins in the blood.
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