Biomedical Engineering Reference
In-Depth Information
typical artery, 0.001 in a capillary, and 400 for a typical vein. This corresponds to turbulent
flow in the aorta (where the aortic wall is strengthened to overcome the turbulent forces),
laminar flow in arteries and veins, and creeping flow in capillaries. Thus, there are added
forces, mixing, and momentum transport in the aorta and virtually no momentum transport
in capillaries. However, as was noted in the discussion on mass transfer in systemic capil-
laries, it is the low axial blood flow that allows for the radial mass transfer to occur. This
could not happen in arteries, since the fluid velocity is too large to allow any significant
radial mass transfer to occur. However, with turbulent flow in the aorta, there is consider-
able mixing of fluid layers and added wall shear stresses. As a result, there is added mass
transfer across the artery walls, which is why arterial disease is prominent in the aorta. Such
a disease is caused by mass transfer of lipids across the arterial wall and by distortion of the
endothelial cells lining the vessel wall, allowing for such mass transfer to occur in the gaps.
A depiction of the systemic capillaries is shown in Figure 14.39. The Reynolds number is
less than 1 and the flow is creeping flow. Such flow produces entirely viscous flow with no
inertial effects, no turbulence, and no flow separation. As such, there are no negative effects
due to the circuitous nature of the capillary bed nor to sudden sharp turns or bifurcations.
In larger vessels, such vessel geometry would produce turbulence and/or flow separation.
14.2.7 Blood Pressure Measurement
The measurement of blood pressure noninvasively is conducted by the well-known
blood pressure cuff assembly: the sphygmomanometer. Most individuals have had
their blood pressure taken with this mechanism. It involves inflating a pressure cuff just
above the elbow to a pressure significantly exceeding systolic blood pressure (120 mm Hg).
At such a high pressure, the artery just below the surface (brachial artery) collapses as the
externally applied pressure from the cuff exceeds the internal blood pressure inside the
artery. A stethoscope is placed just below the cuff. As the pressure is reduced, the external
pressure will eventually fall slightly below the systolic blood pressure, and the brachial
artery will open slightly. The blood at that high pressure will jet through the small opening,
producing turbulent flow. Such jetting turbulent flow can be heard via the stethoscope.
This is similar to a heart murmur heard through a stethoscope, which is indicative of a leaky
heart valve, also producing turbulence, as well as separated flow downstream of a stenosis.
FIGURE 14.39
The network of the systemic capillaries leading from arterioles and ending in venules, with
bending and bifurcating elements in the capillary bed that are possible with creeping flow.
