20
Dynamics
of
the Vascular System
and are called arteriovenous capillaries. The capillary and venule have
very thin walls. The capillary as mentioned before, lacks smooth muscle
and only has
a
layer of endothelium. The smooth muscle and elastic
tissue are present in greater amounts in vessels having vasoactive
capabilities, such as arterioles. This is also the site of greatest drop in
mean blood pressure.
For this reason, arterioles are the principal
contributors to peripheral vascular resistance that can effectively alter
cardiac output.
The structural components of the microcirculation are classified into
resistance, exchange, shunt, and capacitance vessels.
The resistance
vessels, comprising
the arterioles,
metarterioles, and precapillary
sphincters, serve primarily to decrease the arterial pressure to the levels
of the capillaries to facilitate effective exchange.
2.2
Mechanical Properties
of Blood
Vessels
2.2.1
Some Geometric Aspects
of
Blood Vessels
The arterial system
is
a tapered branching system. Changes in lumen
size are often associated with branching and appropriate tapering. In the
normal arterial system, the branched daughter vessels are always
narrower than the mother vessel, but with slightly larger total cross-
sectional areas. This means that the branching area ratio, or the ratio of
the total cross-sectional area of the daughter vessels to that of the mother
vessel, is slightly greater than one. This has significance in terms of
pulsatile energy transmission.
Arterial diameters and lumen areas of the vascular tree can be
determined from postmortem cast or angio-radiography. Arteries in man
and in dog retract some
25
to 40 percent when removed. It is therefore
necessary that in-vivo lengths are restored and corresponding pressures
are given. Under normal conditions, a higher distending pressure lead
to
greater lumen diameter. Arterial vessel dimensions have been provided
for the dog (McDonald, 1974) and man (Westerhof et al., 1969). The
latter were used for constructing the analog model of the human systemic
arterial tree.
