CHAPTER 5 • VASCULAR FUNCTION
103
Laminar Flow
Turbulent Flow
■ FIGURE 5.8 Laminar versus turbulent flow. In
laminar flow, blood flows sm oothly in concentric
layers parallel w ith the axis of the blood vessel,
w ith the highest velocity in the center of the ves-
sel and the lowest velocity next to the endothelial
lining of the vessel. When laminar flow becomes
disrupted (e.g., by a atherosclerotic plaque), it
becomes turbulent; blood no longer flows in
concentric, parallel layers, but rather moves in dif-
ferent paths, often form ing vortices.
Turbulence causes increased energy loss
and a greater pressure drop along a ves-
sel length than predicted by the Poiseuille
relationship (Equation 5-7). For example,
as illustrated in Figure 5.9, if blood flow is
increased twofold across a stenotic arterial
segment that already has mild turbulence,
the pressure drop across the stenosis may
increase threefold or fourfold, and the turbu-
lence enhanced. The Poiseuille relationship
predicts a twofold increase in the pressure
drop across the lesion because the pressure
drop is proportionate to flow under laminar
flow conditions (see Fig. 5.10). Turbulence,
however, alters the relationship between flow
and perfusion pressure so that the relation-
ship is no longer linear and proportionate
as described by the Poiseuille relationship.
Instead,
a
greater
perfusion
pressure
is
required to propel the blood at a given flow
rate when turbulence is present. Alternatively,
a given flow causes a greater pressure drop
Normal Flow
A P = 10 mmHg
2-Times Normal Flow
A P = 35 mmHg
■ FIGURE 5.9 Effects of flow on turbulence.
A tw ofold increase in flow across a stenotic lesion
causes a disproportionate increase in the pressure
drop (AP) across the lesion due to increased tu r-
bulence. In this illustration, AP may increase three-
fold or fourfold instead of tw ofold as predicted by
Poiseuille relationship when flow is doubled.
across a resistance than predicted simply by
the radius and length of the resistance ele-
ment because of increased energy losses asso-
ciated with turbulence.
Series and Parallel Arrangement
of the Vasculature
It is crucial that Poiseuille’s equation should
be applied only to single vessels.
If,
for
example, a single arteriole within the kid-
ney were constricted by 50%, although the
resistance of that single vessel would increase
16-fold, the vascular resistance for the
entire
renal circulation would not increase 16-fold.
The change in overall renal resistance would
be so small that it would be immeasurable.
This is because the single arteriole is one of
many resistance vessels within a complex net-
work of vessels, and therefore, it constitutes
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