Vascular Biology, Structure
and
Function
19
Veins have
a
greater total number than arteries and thus the venous
system has a much larger cross-sectional area. This results in
a
much
larger volume available for blood storage. Indeed, veins are known as
low pressure storage reservoirs of blood.
Under normal physiological
conditions, the venous system contains about
75%
of the total blood
volume in the systemic circulation with the systemic arterial system
constitutes some
15%.
For this reason, veins are often referred to as
capacitance vessels.
Venous return is an important determinant of
cardiac output.
The pulmonary circulation contains about one quarter
the blood volume of the systemic circulation.
Veins have much thinner walls and less elastins than arteries.
Because of this, veins are stiffer than arteries.
However, the low
operating pressure and collapsibility allows veins to increase their
volume by several times under a small increase of distending pressure.
There are bicuspid
valves in veins.
These valves
permit
unidirectional flow, thus preventing retrograde blood flow to tissues due
to high hydrostatic pressures. These valves are notably present in the
muscular lower limbs.
2.1.5
The Microvasculature
As
stated previously, the function of the cardiovascular system is to
provide a homeostatic environment for the cells
of
the organism. The
exchange of the essential nutrients and gaseous materials occurs in the
microcirculation at the level
of
the capillaries. These microvessels are
of
extreme importance for the maintenance of
a
balanced constant cellular
environment.
Capillaries and venules are known
as
exchange vessels
where the interchange between the contents in these walls and the
interstitial space occur across their walls.
The microcirculation can be described in terms of a network such as
that shown in Fig. 7.1.1.
It consists of an arteriole and its major
branches, the metarterioles. The metarterioles lead to the true capillaries
via
a
precapillary sphincter.
The capillaries gather to form small
venules, which in turn become the collecting venules. There can be
vessels going directly from the metarterioles to the venules without
supplying capillary beds. These vessels
form
arteriovenous (A-V) shunts
