140
Dynamics
of
the Vascular System
facilitates early rapid ventricular ejection.
During mid-systole, arterial
compliance begins to decline, this corresponds to increased aortic
pressure and reduced ejection after peak ventricular outflow. In the late
systolic phase, arterial compliance declines rapidly with
a
concurrent
rapid decline of aortic flow, despite a falling aortic pressure.
The
compliance value is much higher at early systole than at late systole.
Arterial system compliance reaches its minimum
at
the end of the
ejection. For the diastolic period, when aortic flow is zero, compliance
follows an exponential relation as given by equation
(4.5.64).
Its value
increases throughout the diastole towards maximum, readying for the
following ventricular ejection.
Time
(sec)
Fig.
4.5.1
1
:
Nonlinear pressure-dependent characteristics
of
arterial compliance
as
a
function
of
time plotted for a complete cardiac cycle.
Arterial compliance increases
initially at the beginning
of
ejection and declines with increasing pressure. It reaches a
minimum at about end-systole and increases steadily thence towards the end of diastole.
In describing fluid motions with a linearized Navier-Stokes equation,
nonlinear terms are assumed to be small and are consequently neglected.
Ling et
al.
(1
973)
Computed flows from an accurately measured pressure
gradient (with a claimed resolution of
0.001
mmHg). They found that
linear theory overestimated the steady flow term by several folds, while
the
pulsatile
flow
waveforms
conformed
rather
well
with
