12
Dynamics
of
the Vascular System
by which pressure and flow pulses propagate and reflect can also be
quantified.
Chapter
5
addresses the vascular branching aspects of the circulation,
whether of arterial, venous or capillary, except the latter
two
are dealt in
more detail in subsequent chapters. Branching geometry is examined in
terms of morphological measurements. The basic fluid mechanic aspects
of vascular branching in terms of pressure and flow transmission, shear
stresses
are
explained, best with
illustrations and mathematical
formulations.
How
efficient the pressure and flow pulses transmit depends on the
propagation and reflection characteristics through different arteries and
vascular branching junctions.
Pulse wave velocity, a popularly used
index to describe the vascular stiffness, is dependent on the geometric
and
elastic properties
of
the local arterial wall.
Its
measurement
is
therefore, elaborated.
With differing vascular impedances, wave reflections arise, because
of the mismatching in impedances. The large peripheral resistances in
the arterioles are the principal sites contributing to reflections. Increased
wave reflection increases blood pressure amplitude and thus decreases
flow. This reduces the pulse transmission efficiency for the propagating
pulse.
Pulse transmission through vascular branching junctions is
dictated by the local blood vessel properties. For forward traveling wave,
it is practically impedance-matched, resulting in optimal transmission.
For the backward traveling wave towards the heart,
it is greatly
attenuated at the vascular branching. Thus, the design of the arterial tree
is to facilitate pulse transmission to vascular beds.
How this is
optimized is explained.
Chapter
6
deals with the
less
studied aspects of the venous
circulation, because of its low pressure and collapsibility and less life-
threatening behavior.
Blood volume is the highest at rest in the venous
circulation, giving rise to its reservoir-like properties. The functional
aspects
of
collapsibility and venous valves are also discussed, in terms
of
pressure-flow relations and the waterfall hypothesis. Modeling aspect is
given in terms of mathematical descriptions and hydrodynamic set-ups.
Chapter
7
deals with the microcirculation. The greatest drop
in
mean
blood pressure
is
found in the arterioles, hence justifying the vascular
