that volume toward the heart. In addition, the skeletal muscle
and respiratory pumps are more active with exercise, increasing
venous return and cardiac output.
The nervous system plays a key role in the circulatory
responses to exercise. In fact, the mere anticipation of exercise
can induce certain circulatory responses before an individual
even begins to exercise. For example, increased sympathetic and
decreased parasympathetic input to the heart from the medullary
cardiovascular center enhances cardiac output prior to exercise.
With exercise, increased sympathetic output to the arterioles
lowers total peripheral resistance and increased sympathetic
output to the veins increases venous return with exercise.
Local mechanisms play a role as well. As activity increases,
the partial pressure of oxygen of the exercising tissue decreases
and the partial pressure of carbon dioxide and acidity levels
increases. These changes signal a need for increased blood flow
to these regions. Vasodilation of the blood vessels in these
tissues allows for increased flow to meet the change in
oxygen demand. In addition, the Bohr effect will enhance the
unloading of oxygen from hemoglobin circulating through
these tissues. Figure 8.4 summarizes many of the short-term
changes in circulatory function that accompany exercise.
With years of physical exercise, long-term changes in
circulatory function can occur. Training increases an individ-
ual’s maximum cardiac output, the factor which most often
limits the ability of the body to meet the increased oxygen
demands required to meet an increase in workload. Long-term
physical training has a significant impact on stroke volume.
An increased stroke volume is achieved through enhanced
pumping ability of the heart. The ventricular walls thicken,
contract more forcibly, and eject more blood with each heart
beat. As a consequence, the degree to which the heart rate is
elevated for a given workload is reduced in trained athletes
(Figure 8.5). Venous return is enhanced as well through an
overall increase in blood volume.
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