174
Dynamics
of
the
Vascular
System
6.1.2
Structural Properties of Veins
Structurally speaking, veins and arteries have the same histological
components as arteries, but with different contents and composition. The
relatively thin wall of the veins, also contributes greatly to the large
observed compliance.
The collapsibility of the vein is due to several
factors including: thin-walled vessel, large compliance, low transmural
pressure. The latter
is
the principal controlling factor,
as
we shall see in
the next section.
Despite the differences between arteries and veins, saphenous vein is
the most commonly utilized vessel in coronary artery bypass surgery.
The difference in elastic properties between arteries and veins is well
appreciated from their differences in distending pressure and wall
structure.
Venous walls are much thinner than those of correspondingly sized
arteries, and are truly thin-walled vessels, i.e. h/r<O.l
.
They contain
much less smooth muscle and less elastic than arteries. Because of the
collagen elastin composition, they exhibit less elastic recoil, but are
easily
stretched.
Short-term venous blood redistribution
can be
accomplished by smooth muscle tone, or activation of sympathetic nerve
fibers imbedded in the venous walls.
Smooth muscle activation can
alter the underlying elastic properties.
Veins are known to be non-circular in cross-section. Its collapsibility
has been debated as to its inefficiency in metabolite transportation and
blood flow. It has been shown that for an efficient fluid flow, the vessel
needs to be cylindrical. This implies a circular cross-sectional area.
Veins however, often exhibit an elliptical cross-sectional area. With an
eccentricity ratio of
2
(major axis diameter/minor axis diameter), the
power required to deliver the same amount of blood is almost twice
(125/64) that for a circular vessel lumen. In other words, for a given
amount of power, the blood flow through an elliptical vessel lumen is
about half (64/125) that of a circular lumen. However, this inefficiency is
well made up by the presence of venous valves. These valves serve as
auxiliary
one-way
facilitators
that
reduce
backflow
which
are
energetically wasteful.
