200
Dynamics
of
the Vascular System
The primary role that the capillaries play
is
in the oxygen exchange.
The quantity of oxygen extracted from the blood during its flow through
capillaries is determined by the relationship between the rate of oxygen
utilization and the blood flow. This is illustrated in Fig.
7.1.5.
The top
figure shows slight oxygen extraction and small arterio-venous oxygen
differences occur in tissues with relatively small oxygen requirements
and active blood, such as skin. The bottom figure shows tissues which
release energy at rapid rates, such as contracting muscle, extract a major
portion of the oxygen from the blood.
As
we mentioned earlier, capillaries in some tissues may consist of
arterio-venous
(A-V)
capillaries known as thoroughfare channels and
true capillaries.
The blood flow through different portions of the
capillary bed
is
affected by contraction and relaxation of smooth muscle
in the arterioles,
A-V
capillaries and precapillary sphincters.
Phasic
changes in these regions produce cyclic alterations in the amount and
distribution of blood flow through the various capillaries. This is known
as the vascular vasomotion (Fig.
7.1.6).
7.2.
Pressure-Flow Relation and Microcirculatory Mechanics
7.2.1
Flow-Related Mechanical Characteristics
of
the Microcirculation
Pressure, flow and forces are drastically smaller in the microcirculation
as opposed to the macro-circulation. We know that the Reynolds number
as
defined by
(7.2.1)
is very small as compared an artery. For instance, the Reynolds number
may be
0.03
for a
100
pm arteriole.
With even smaller diameter and
blood velocity, the Reynolds number is at least an order smaller in the
capillary.
Steady flow
is
assumed in many hemodynamic and rheological
studies. This
is
because the Womersley’
s
number, defined as
