204
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
rate is initially lower in the supine position,
the percent increase in heart rate is greater in
the supine position, which compensates for
the reduced ability to increase stroke volume.
Overall, the change in cardiac output during
exercise, which depends upon the fractional
increases in both stroke volume and heart
rate, is not appreciably different in the supine
versus standing position.
Physical conditioning permits a person to
achieve a higher cardiac output, whole-body
oxygen
consumption,
and workload
than
a person who has a sedentary lifestyle. The
increased cardiac output capacity is a conse-
quence, in part, of increased ventricular and
atrial responsiveness to inotropic stimulation
by sympathetic nerves. Conditioned individu-
als also have stronger, hypertrophied hearts,
much like what happens to skeletal muscle
in response to weight training. Coupled with
enhanced
capacity
for
promoting
venous
return by the muscle pump system, these
cardiac changes permit highly conditioned
individuals to achieve ventricular ejection
fractions that can exceed 90% during exer-
cise. In comparison, a sedentary individual
may not be able to increase ejection fraction
above 75%.
In a conditioned individual, resting heart
rate is lower and resting stroke volume is
higher than in a sedentary person— resting
cardiac output is not necessarily different.
Because the maximal heart rate of a condi-
tioned individual is similar to that of a sed-
entary individual of the same age, the lower
resting heart rates of a conditioned person
allow for a greater percent increase in heart
rate during exercise. This greater capacity to
increase heart rate, coupled with a greater
capacity to enhance stroke volume, permits
a conditioned individual to achieve maximal
cardiac outputs that can be 50% higher than
those found in sedentary people. Another
important distinction between a sedentary
and conditioned person is that for a given
workload, the conditioned person has a lower
heart rate. Furthermore, a conditioned per-
son is able to sustain higher workloads for a
longer duration and recover from the exercise
much more rapidly.
Environmental conditions can significantly
alter
cardiovascular
responses
to
exercise.
High altitudes, for example, decrease maximal
stroke volume and cardiac output. The reason
for this is that arterial PO2 and oxygen content
are reduced at higher elevations because of
decreased atmospheric pressure. This decreases
oxygen delivery to tissues, particularly to con-
tracting muscle (both skeletal and cardiac),
thereby
resulting
in
insufficient
oxygena-
tion at lower workloads. Myocardial hypoxia
decreases
maximal
inotropy,
which
results
in reduced stroke volume. Reduced oxygen
delivery to exercising muscle reduces exercise
capacity in the muscle and results in increased
production of lactic acid as the muscle switches
over to anaerobic metabolism in the absence of
adequate oxygen; that is, the anaerobic thresh-
old is reached at a lower workload.
Increased temperature and humidity affect
cardiovascular responses during exercise by
diverting a greater fraction of cardiac output
to the skin to enhance heat removal from
the body. This decreases the availability of
blood flow for the contracting muscles. With
elevated temperature and humidity, maximal
cardiac output and oxygen consumption are
reached at lower workloads, thereby reducing
exercise capacity as well as endurance. Fur-
thermore, dehydration can accompany high
temperatures.
Dehydration
reduces
blood
volume and central venous pressure, which
attenuates the normal increase in cardiac out-
put associated with exercise. This can lead
to a fall in arterial pressure and heat exhaus-
tion. Signs of heat exhaustion include general
fatigue, muscle weakness, nausea, and mental
confusion; it usually results from dehydration
and loss of sodium chloride associated with
physical activity in a hot environment— core
temperature is not necessarily elevated.
Increased
age
reduces
maximal
exer-
cise
capacity.
Maximal
oxygen
consump-
tion decreases about 40% between 20 and
70 years of age. There are many reasons for
this decline. With increasing age, maximal
heart rate decreases. Maximal heart rate is
approximately 220 beats/min minus the age
of a person. Therefore, the maximal heart rate
of a 70-year-old person is about 25% lower
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