Human Physiology, 14th edition (2016)

472 Chapter 14

proportionate increase in cardiac output and achieve a higher

absolute cardiac output during exercise. This large cardiac out-

put is the major reason for the improved oxygen delivery to

skeletal muscles that occurs as a result of endurance training.

During exercise, the cardiac output can increase fivefold—
from about 5 L per minute to about 25 L per minute. This is
primarily due to an increase in cardiac rate. The cardiac rate,
however, can increase only up to a maximum value ( table 14.6 ),
which is determined mainly by a person’s age. In well-trained
athletes, the stroke volume can also increase significantly,
allowing these individuals to achieve cardiac outputs during
strenuous exercise up to six or seven times greater than their
resting values. This high cardiac output results in increased
oxygen delivery to the exercising muscles; this is the major rea-
son for the much higher than average maximal oxygen uptake
( max) of elite athletes (chapter 12).
In most people, the increase in stroke volume that occurs
during exercise will not exceed 35%. The fact that the stroke
volume can increase at all during exercise may at first be surpris-
ing, given that the heart has less time to fill with blood between
beats when it is pumping faster. Despite the faster beat, however,
the end-diastolic volume during exercise is not decreased. This
is because the venous return is aided by the improved action of
the skeletal muscle pumps and by increased respiratory move-
ments of the diaphragm during exercise ( fig.  14.21 ). The end-
diastolic volume is not significantly changed during exercise,
so any increase in stroke volume that occurs must be due to an
increase in the proportion of blood ejected per stroke.
The proportion of the end-diastolic volume ejected per
stroke can increase from 60% at rest to as much as 90% dur-
ing heavy exercise. This increased ejection fraction is produced
by the increased contractility that results from sympathoadrenal
stimulation. There also may be a decrease in total peripheral
resistance as a result of vasodilation in the exercising skeletal
muscles, which decreases the afterload and thus further aug-
ments the increase in stroke volume. The cardiovascular changes
that occur during exercise are summarized in table 14.7.
Endurance training often results in a lowering of the resting
cardiac rate and an increase in the resting stroke volume. The
lowering of the resting cardiac rate results from a greater inhi-
bition of the SA node by the vagus nerve. The increased resting
stroke volume is believed to be due to an increase in blood
volume; indeed, studies have shown that the blood volume
can increase by about 500 ml after only eight days of training.
These adaptations enable the trained athlete to produce a larger

Figure 14.21 Cardiovascular adaptations to

exercise. These adaptations (1) increase the cardiac output,

and thus the total blood flow; and (2) cause vasodilation in the

exercising muscles, thereby diverting a higher proportion of the

blood flow to those muscles.

Cardiac

output

Cardiac

rate

Stroke

volume

Sympathoadrenal

system Improved

venous

return

Deeper

breathing

Skeletal

muscle

activity

Metabolic

vasodilation

in muscles

Sympathetic

vasoconstriction

in viscera

Blood flow to

skeletal muscles

Table 14.6 | Relationship Between Age
and Average Maximum Cardiac Rate*

Age Maximum Cardiac Rate

20–29 190 beats/min

30–39 160 beats/min

40–49 150 beats/min

50–59 140 beats/min

(^601) 130 beats/min
*Maximum cardiac rate can be estimated by subtracting your age from 220.

Clinical Investigation CLUES

Mark trained to run marathons.

• What happened to his cardiac output and blood
  flow through his brain, heart, and skeletal muscles
  during these sessions?


• How might such endurance training affect his
  resting cardiac rate and stroke volume?

| CHECKPOINT

8. Describe blood flow and oxygen delivery to the
   myocardium during systole and diastole.
   9a. State how blood flow to the heart is affected by
   exercise. Explain how blood flow to the heart is
   regulated at rest and during exercise.
   9b. Describe the mechanisms that produce vasodilation
   in exercising muscles, and list the cardiovascular
   changes that contribute to increased blood flow to
   exercising muscles.
   9c. Explain how the stroke volume can increase during
   exercise despite a shorter time available for the heart
   chambers to fill with blood.