The
Venous
System
175
6.1.3
Venous Return
The heart
fills
with blood during diastole.
Thus, the amount of
venous return and filling pressure are both important governing factors of
adequate filling and subsequent ejection. Superior and inferior vena cava
are the principal large veins that return blood to the heart. Being in the
thorax, the intrathoracic pressure is also an importance consideration.
A
positive filling pressure must be maintained in these veins to facilitate
venous return. This is necessarily
so
independent of body position,
magnitude and distribution of blood volume.
Maintaining this proper
central venous pressure requires the venous system to rapidly adjust to
change in blood volume and its distribution. Since the vena cavae return
blood to the right atrium, it is important that the venous pressure is
slightly above the right atrial pressure. This
is
also necessary to prevent
collapse when the difference of intravascular pressure and extravascular
pressure becomes essentially zero or negative
AP=q-p,
(6.1.2)
Right atrial pressure is often considered as the “preload” to the pumping
function of the heart. It is also often referred to
as
the filling pressure.
Normal filling pressure is below
12
mmHg. Extremely high right atrial or
filling pressure thus can hinder venous return. In coronary heart diseased
subjects, it is not unusual to find filling pressure that exceeds
30
mmHg.
During exercise, the increased cardiac output depends also on an
increased amount
of
venous return to the heart.
Reduced filling due to
lower venous return can result in reduced stroke volume, reflected in the
Starling’s law. It is recognized that increased sympathetic smooth muscle
activation in the venous walls or venous tone in conjunction with
compression of veins by surrounding skeletal muscles are important,
together with the unidirectional venous valves, in returning blood
towards the heart.
