The
Microcirculation
20
1
(7.2.2)
is very small.
Womersley’s number can be viewed as the ratio of
oscillatory
flow to steady flow. When
aw
approaches zero, the
approximation
of
a steady flow is a reasonable one.
In relation to entry length, it is now small compared with the diameter
of the microvessels. Thus, the flow becomes fully developed towards a
parabolic profile, despite the relatively short length of the vessels.
The Newtonian fluid aspect needs to be carefully addressed.
The
blood and its formed elements now contribute more to the abnormality of
the viscosity. The well-known Fahraeus-Lindqvist effect explains the
decreasing blood viscosity
in
very small vessels. Nevertheless, the
Poiseuille’s
equation
has
been
viewed
as
important
in
the
microcirculation, in that the flow is determined by the driving force of
the pressure gradient and the resisting force of viscosity.
(7.2.3)
Although
remaining
pulsatile, the flow in the capillaries
is
intermittent at best.
A
single arteriole branches into several and
sometimes
15
or so capillaries.
The
branching
phenomena
and
optimization aspect we discussed in Chapter
5,
now needs to incorporate
considerably more local control aspects, in terms
of
exchange between
the capillaries and its surrounding tissues, as we11 as the vasomotion due
to smooth muscle tone in the arterioles.
The flow into the capillaries has been shown to remain pulsatile or
intermittent in nature.
It has also been shown that the rhythmic
vasomotor activity of the precapillary sphincters is responsible for the
observed intermittency. The sphincters may also exhibit constriction and
dilation in response to changes in local metabolites, chemicals,
or
sympathetic stimuli.
Together with the arterioles, the precapillary
