Chapter
6
The
Venous
System
6.1
The Reservoir Properties and Venous Return
6.1.1
Venous Compliance and Reservoir Characteristics
Veins are the principal conduits by which deoxygenated blood is returned
to the heart, thus together with the arterial system, completing the closed-
loop feature of the cardiovascular system.
Blood from capillaries are
returned through collecting venules to small veins and to large veins.
Except the largest veins, i.e. vena cava, the great pulmonary veins and the
smallest venules, veins have valves whose primary function is to
facilitate the return of blood to the heart and prevent backflow.
The reservoir properties of the veins can be easily appreciated, since
more than
70%
of the total blood volume of the systemic vascular system
are contained in veins under normal conditions,
as
we have seen in Fig.
2.1.2.
Hemodynamically
speaking, this
is because of the large
compliance, due to a large incremental volume (dV) and small pulsatile
change in pressure (dP),
dV
C=-
dP
(6.1.S)
The reservoir property can also be easily appreciated in the case of
blood loss.
Under such condition, venous blood volume, not arterial
blood volume, decreases in order to maintain sufficient arterial perfusion
pressure. This is accomplished by the innervation of sympathetic fibers
lining the venous walls, stimulation of which causes vasoconstriction,
