104
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Flow
■ FIGURE 5.10 Effects of turbulence on the pressure-flow relationship. Turbulence decreases flow at any
given perfusion pressure, or requires a greater perfusion pressure to drive a given flow.
only a small fraction of the resistance for the
whole organ. To help understand this com-
plex arrangement of vessel architecture, it is
necessary to examine the vascular compo-
nents in terms of series and parallel elements.
The parallel arrangement of organs and
their circulations (see Fig. 1.2) is important
because
parallel vessels decrease total vascular
resistance
. When there is a parallel arrange-
ment of resistances, the reciprocal of the total
resistance is equal to the sum of the reciprocals
of the individual resistances. For example, the
total resistance (RT) of three parallel resistances
(R:, R2, R3) would be
1
1
1
1
=
+
+
RT
R1
R2
R3
or,
solving for RT,
R -
1
Eq.5-8
T
1
1
1
+
+
R1
R2
R3
Two
important
principles
emerge
from
Equation 5-8. First,
the total resistance of
a network of parallel resistances is less than
the resistance of the single lowest resistance;
therefore,
parallel
vessels
greatly
reduce
resistance. For example, assume that R2
= 5,
R2 = 10, and R3 = 20. When the equation is
solved, RT
= 2.86, a value that is less than the
lowest individual resistance. The resistance
calculation for parallel networks explains
why capillaries constitute a relatively small
fraction of the total vascular resistance of an
organ or microvascular network. Although
capillaries have the highest resistance of
individual vessels because of their small
diameter, they also form a large network of
parallel vessels. This reduces their resistance
as a group of vessels. The second principle is
that
when many parallel vessels exist, chang-
ing the resistance of a small number of these
vessels will have little effect on total resistance.
Within an organ, the vascular arrangement
is
a
combination
of
series
and
parallel
elements. In Figure 5.11, the artery, arteri-
oles, capillaries, venules, and vein as groups
of vessels are in series with each other. All
of the blood that flows through the artery
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