The
Venous System
173
and the narrowing
of
lumen diameters, hence a reduction in volume of
the venous reservoir.
Veins return blood from organ vascular bed capillaries to the heart.
The principal features of the venous system are its large volume and low
pressure, hence greater compliance. Venous pressures are generally low,
seldom exceed
12
mm Hg, or
10%
of the systolic pressure in arteries, i.e.
120
mmHg. Veins have considerably smaller wall-thickness-to-radius
ratio than corresponding arteries. This gives rise to the reservoir
properties
of
the veins. In comparison, the walls of veins are much
thinner, with less smooth muscle cells, and are less stiff than arterial
walls. The large diameter and low pressure of veins permits the venous
system to function as a storage reservoir for blood. But the relatively
weak wall also means veins can be more easily suffering from tearing
under greater shear stress or pressure.
During hemorrhage and blood loss events, venous blood volume, not
arterial blood volume decreases in order to maintain vascular blood flow.
This is accomplished by a reduction in total venous volume. This
is
achieved by vasoconstriction-induced reduction in vessel size of the vein
whose walls have smooth muscle cells, as well as sympathetic adrenergic
nerve fibers. Thus, the interplay of reflex action and the volume
adjustment, allow venous pressure also to be maintained. The volume
loss
is
normally restored through fluid retention and replenishment.
Veins acting as
a
storage reservoir
of
blood, also exhibit the function
of
blood distribution. Thus, they provide a critical element in blood
volume control. For instance, in muscular parts of the body, such as
upper and lower limbs, venous return is aided by the increase in venous
tone ensures that the skeletal muscle pump increases venous pressure to
ensure returning of blood to the heart.
This is in addition to the
peristaltic action
of
the veins and the one-way flow valves within the
veins. The failure of the skeletal muscle contraction can lead to the
common phenomenon of “venous pooling’ of blood in the venous system
of these limbs. Veins also play a major role in body organ temperature
control through countercurrent exchanger mechanism. This latter is
particularly effective in smaller vessels.
