110
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
and stroke volume by the Frank-Starling
mechanism. Left ventricular stroke volume
subsequently falls because of reduced pulmo-
nary venous return to the left ventricle; the
reduced stroke volume causes cardiac out-
put and arterial blood pressure to decrease. If
systemic arterial pressure falls by more than
20 mm Hg upon standing, this is termed
orthostatic or postural hypotension. When
this occurs, cerebral perfusion may fall and a
person may become “light headed” and expe-
rience a transient loss of consciousness (syn-
cope). Normally, baroreceptor reflexes (see
Chapter 6) are activated to restore arterial
pressure by causing peripheral vasoconstric-
tion and cardiac stimulation (increased heart
rate and inotropy). Furthermore, as seen in
the next section, limb movement (e.g., walk-
ing) in the upright position facilitates venous
return, thereby lowering venous pressures in
the leg and partially restoring CVP.
The influence of gravity on venous volume
can be visualized by looking at the veins on
the back of the hand when below the level
of the heart and then raising the hand above
the head. When this is done, the veins col-
lapse because they are above the level of the
heart and the negative hydrostatic pressure
caused by gravity acting on the column of
blood reduces the pressure within the veins.
If the hand is suddenly brought down below
the heart, the veins will refill with blood as
the positive hydrostatic pressure increases the
venous pressure and distends the veins.
SKELETAL MUSCLE PUMP
Veins,
particularly in
extremities,
contain
one-way
valves
that
permit
blood
flow
toward the heart and prevent retrograde flow.
Deep veins in the lower limbs are surrounded
by large groups of muscle that compress the
veins when the muscles contract. This com-
pression increases the pressure within the
veins, which
closes
upstream
valves
and
opens downstream valves, thereby function-
ing as a pumping mechanism (Fig. 5.15). This
pumping mechanism plays a significant role
in facilitating venous return during exercise.
Rhythmical contraction of leg muscles also
Relaxed
Contracted
■ FIGURE 5.15 Rhythmic contraction of skeletal
muscle compresses veins, particularly in the lower
limbs, and propels blood toward the heart through
a system of one-way valves.
helps to counteract gravitational forces when a
person stands up by facilitating venous return
and lowering venous and capillary pressures
in the feet and lower limbs. For example,
human experiments have shown that when a
person is standing still, the venous pressure
measured at the ankle is about 90 mm Hg.
When the person begins to walk, the venous
pressure falls to 30 to 40 mm Hg after sev-
eral steps. When the venous valves become
incompetent, as occurs when veins become
enlarged (varicose veins), muscle pumping
becomes ineffective. Besides the loss of mus-
cle pumping in aiding venous return, blood
volume and pressure increase in the veins of
the dependent limbs, which increases cap-
illary pressure and may cause edema (see
Chapter 8).
RESPIRATORY ACTIVITY
(ABDOMINOTHORACIC OR
RESPIRATORY PUMP)
Venous return to the right atrium from the
abdominal vena cava is determined by the
pressure difference between the abdominal
vena cava and the right atrial pressure, as well
as by the resistance to flow, which is primar-
ily determined by the diameter of the thoracic
vena cava. Therefore, increasing right atrial
pressure
impedes
venous
return,
whereas
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