Cardiovascular Physiology
415
out of the capillaries into the interstitial space. This is why our
feet swell during prolonged standing. The combined effects
of venous pooling and increased capillary fi ltration reduce the
effective circulating blood volume very similarly to the effects
caused by a mild hemorrhage. This explains why a person may
sometimes feel faint upon standing up suddenly. This feeling is
normally very transient, however, because the decrease in arte-
rial pressure immediately causes refl ex baroreceptor-mediated
compensatory adjustments virtually identical to those shown in
Figure 12–56 for hemorrhage.
The effects of gravity can be offset by contraction of the
skeletal muscles in the legs. Even gentle contractions of the leg
muscles without movement produce intermittent, complete
emptying of the leg veins so that uninterrupted columns of
venous blood from the heart to the feet no longer exist (
Figure
12–60
). The result is a decrease in both venous distension
and pooling plus a signifi cant reduction in capillary hydro-
static pressure and fl uid fi ltration out of the capillaries. This
phenomenon is illustrated by the fact that soldiers may faint
while standing at attention for long periods of time because
of minimal leg muscle contractions. Here, fainting may be
considered adaptive because the venous and capillary pressure
changes induced by gravity are eliminated once the person is
horizontal. The pooled venous blood is mobilized, and the
fi l t e r e d fl uid is absorbed back into the capillaries. Thus, the
wrong thing to do for a person who has fainted is to hold him
or her upright.
Exercise
During exercise, cardiac output may increase from a resting
value of 5 L/min to a maximal value of 35 L/min in trained
athletes.
Figure 12–61
illustrates the distribution of this car-
diac output during strenuous exercise. As expected, most of the
increase in cardiac output goes to the exercising muscles, but
there are also increases in fl
ow to skin if it beomes necessary
to dissipate heat, and to the heart, required for the additional
work performed by the heart in pumping the increased cardiac
output. The increases in fl ow through these three vascular beds
are the result of arteriolar vasodilation in them. In both skeletal
and cardiac muscle, local metabolic factors mediate the vaso-
dilation, whereas the vasodilation in skin is achieved mainly
by a decrease in the fi ring of the sympathetic neurons to the
skin. At the same time that arteriolar vasodilation is occurring
Heart
Veins
Muscles
Pressure
due to
gravity
= 14 mmHg
Leg muscles
relaxed
Leg muscles
contracted
Pressure
due to
gravity
= 80 mmHg
Figure 12–60
Role of contraction of the leg skeletal muscles in reducing capillary
pressure and fi ltration in the upright position. The skeletal muscle
contraction compresses the veins, causing intermittent emptying so
that the columns of blood are interrupted.
750 (4%)
750 (4%)
12,500 (73%)
1900 (11%)
600 (3%)
600 (3%)
400 (2%)
17,500
Heart
Abdominal
organs
Kidneys
Skin
Other
5000
Total
525 (10%)
1200 (24%)
950 (20%)
430 (9%)
1030 (20%)
Skeletal muscle
650 (13%)
Brain
215 (4%)
Flow
at rest
(ml/min)
Flow during
strenuous exercise
(ml/min)
Figure 12–61
Distribution of the systemic cardiac output at rest and during
strenuous exercise. The values at rest were previously presented in
Figure 12–3.
Adapted from Chapman and Mitchell.
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