"Because of
the hydrostatic pressure, the venous pressure in
the feet would always be about +90mm Hg in a
standing adult were it not for the valves in the
veins. However, every time one moves the legs one
tightens the muscles and compressed the veins
either in the muscles or adjacent to them, and
this squeezes the blood out of the veins.
Yet, the valves in the
veins...are so arranged that the direction of
blood flow can only be toward the heart.
Consequently, every time a person moves the legs
or even tenses the muscles, a certain amount of
blood is propelled toward the heart, and the
pressure in the dependent veins of the body is
lowered. This pumping system is known as the
'venous pump' or 'muscle pump,' and it is
efficient enough that under ordinary
circumstances the venous pressure in the feet of
a walking adult remains less than 22mm Hg.
If the human
being stands perfectly still, the venous pump
does not work, and the venous pressure in the
lower part of the leg can rise to the full
hydrostatic value of 90 mm Hg in about 30
seconds. Under these circumstances the pressure
within the capillaries also increase greatly, and
fluid leaks from the circulatory system into the
tissue spaces. As a result, the legs swell, and
the blood volume diminishes. Indeed, as much as
15 to 20 per cent of the blood volume is
frequently lost from the circulatory system
within the first 15 minutes of standing
absolutely still, as often occurs when a soldier
is made to stand at absolute attention.
VALVE
INCOMPETENCE AND VARICOSE VEINS
The valves of
the venous system frequently become 'incompetent'
or are sometimes destroyed. This is especially
true when the veins have been overstretched by an
excess of venous pressure for a prolonged period
of time, as occurs in pregnancy or when one
stands most of the time (or is not moving one's
legs while sitting). Stretching obviously
increases their cross-sectional areas, but the
valves do not increase in size. Therefore, the
(flaps) of the valves will no longer close
completely and block reverse blood flow in the
enlarged veins. When this develops, the pressure
in the veins of the legs increase still more
owing to the failure of the venous pump; this
further increases the size of the veins and
finally destroys the function of the valves
entirely. Thus, the person develops 'varicose
veins', which are characterized by large, bulbous
protrusions of the veins beneath the skin of the
entire leg and particularly of the lower leg. The
venous and capillary pressure become very high
because of the incompetent venous pump, and the
leakage of fluid from the capillary blood into
the tissue cause constant edema (swelling) in the
legs of these persons when they stand for more
than a few minutes. The edema in turn prevents
adequate diffusion of nutritional materials from
the capillaries to the muscle and skin cells so
that the muscles become painful and weak, and the
skin frequently becomes gangrenous and ulcerates.
Obviously, the best treatment for such a
condition is continual elevation of the legs to a
level at least as high as the heart, but tight
binders on the legs are also of considerable aid
in preventing the edema and its sequelae."
