mechanisms restore sympathetic tone to the
skin and decrease its blood flow. Although
this may help to preserve arterial pressure
temporarily, reduced heat exchange through
the skin can lead to dangerous elevations in
core temperature, resulting in organ damage
and loss of autonomic control. Heat stroke
is a potentially lethal condition that occurs
when core temperatures rise above 105°E
CASE 9-1
A 45-year-old male patient with type 2
diabetes is diagnosed with autonomic
neuropathy, which impairs autonomic
function. He complains of becoming
weak and “light headed” when he
performs physical work such as mowing
the lawn. Explain how this patient’s
autonomic dysfunction may account for
his inability to be engaged in normal
physical activities.
Factors Influencing
Cardiovascular Response
to Exercise
The cardiovascular changes associated with
physical activity are modified by many dif-
ferent factors. The level of activity, which is
commonly expressed as work performed or
whole-body oxygen consumption, affects the
cardiac and vascular responses. Several other
important factors influence cardiovascular
responses at a given workload.
The type of exercise significantly affects
cardiovascular responses. The previous sec-
tion described the cardiovascular responses to
dynamic exercise such as running, walking,
bicycling, or swimming. Dynamic exercise
results in joint movement as muscles contract
rhythmically. In contrast, muscle contraction
without joint movement (isometric or static
contraction) elicits a different cardiovascular
response. An example of this activity would
be trying to lift a very heavy weight at maxi-
mal effort (e.g., bench or leg press).This type
of activity does not incorporate rhythmic
contraction of synergistic and antagonistic
muscle groups; therefore, the muscle pump
system cannot operate to promote venous
return, and so, cardiac output increases rela-
tively little. Furthermore, the abdominotho-
racic pump does not contribute to enhancing
venous return, particularly if the subject
holds his or her breath during the forceful
contraction, effectively performing a Valsalva
maneuver. Unlike dynamic exercise, static
exercise leads to a large increase in systemic
vascular resistance, particularly if a large mus-
cle mass is being contracted at maximal effort.
The increased systemic vascular resistance
results from enhanced sympathetic adrener-
gic activity to the peripheral vasculature and
from mechanical compression of the vascula-
ture in the contracting muscles. As a result,
systolic arterial pressure may increase to over
250 mm Hg during forceful isometric contrac-
tions, particularly those involving large mus-
cle groups. This acute hypertensive state can
produce vascular damage (e.g., hemorrhagic
stroke) in susceptible individuals. In con-
trast, dynamic exercise leads to only modest
increases in arterial pressure.
Body posture also influences how the cardi-
ovascular system responds to exercise because
of the effects of gravity on venous return and
central venous pressure (see Chapter 5). When
a person exercises in the supine position (e.g.,
swimming), central venous pressure is higher
than when the person is exercising in the
upright position (e.g., running). In the resting
state before the physical activity begins, ven-
tricular stroke volume is higher in the supine
position than in the upright position owing
to increased right ventricular preload. Fur-
thermore, the resting heart rate is lower in the
supine position. When exercise commences
in the supine position, the stroke volume can-
not be increased appreciably by the Frank-
Starling mechanism because the high resting
preload reduces the reserve capacity of the
ventricle to increase its end-diastolic volume.
Stroke volume still increases during exercise
although not as much as when exercising
while standing because, in the supine posi-
tion, the increased stroke volume is resulting
primarily from increases in inotropy and ejec-
tion fraction with minimal contribution from
the Frank-Starling mechanism. Because heart
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