Human Physiology, 14th edition (2016)

570 Chapter 16

subjects are within the resting range, these humoral theories

propose that (1) the P^ CO 2 and pH in the region of the chemo-

receptors may be different from these values “downstream,”

where blood samples are taken, and/or (2) that cyclic varia-

tions in these values that cannot be detected by blood samples

may stimulate the chemoreceptors. The evidence suggests that

both neurogenic and humoral mechanisms are involved in the

hyperpnea, or increased total minute volume, of exercise. (The

total minute volume is increased in both hyperpnea and hyper-

ventilation, but in hyperventilation there is also a decrease in

arterial blood P^ CO 2 .)

Lactate Threshold and Endurance Training

The cardiopulmonary system may be unable to deliver adequate

amounts of oxygen to the exercising muscles at the beginning

16.9 EFFECT OF EXERCISE

AND HIGH ALTITUDE ON

RESPIRATORY FUNCTION

The arterial blood gases and pH do not significantly change

during moderate exercise because ventilation increases to

keep pace with the increased metabolism. Adjustments

are also made at high altitude in both the control of ventila-

tion and the oxygen transport ability of the blood.

LEARNING OUTCOMES

After studying this section, you should be able to:

16. Describe the changes in the respiratory system
    that occur in response to exercise training and high
    altitude.


17. Describe the adaptations of the respiratory system
    to living at high altitude.

Changes in ventilation and oxygen delivery occur during
exercise and during acclimatization to a high altitude. These
changes help compensate for the increased metabolic rate dur-
ing exercise and for the decreased arterial P^ O 2 at high altitudes.

Ventilation During Exercise

As soon as a person begins to exercise, breathing becomes
deeper and more rapid to produce a total minute volume that
is many times the resting value. This increased ventilation, par-
ticularly in well-trained athletes, is exquisitely matched to the
simultaneous increase in oxygen consumption and carbon diox-
ide production by the exercising muscles. The arterial blood P^ O 2 ,
P^ CO 2 , and pH thus remain surprisingly constant during exercise
( fig. 16.41 ).
It is tempting to suppose that ventilation increases during
exercise as a result of the increased CO 2 production by the exer-
cising muscles. Ventilation and CO 2 production increase simul-
taneously, however, so that blood measurements of P^ CO 2 during
exercise are not significantly higher than at rest. The mecha-
nisms responsible for the increased ventilation during exercise
must therefore be more complex.
Two kinds of mechanisms— neurogenic and humoral —have
been proposed to explain the increased ventilation that occurs
during exercise. Possible neurogenic mechanisms include the
following: (1) sensory nerve activity from the exercising limbs
may stimulate the respiratory muscles, either through spinal
reflexes or via the brain stem respiratory centers, and/or (2) input
from the cerebral cortex may stimulate the brain stem centers to
modify ventilation. These neurogenic theories help explain the
immediate increase in ventilation that occurs as exercise begins.
Rapid and deep ventilation continues after exercise has
stopped, suggesting that humoral (chemical) factors in the
blood may also stimulate ventilation during exercise. Because
the P^ O 2 , P^ CO 2 , and pH of the blood samples from exercising

Figure 16.41 The effect of exercise on arterial

blood gases and pH. Notice that there are no consistent

or significant changes in these measurements during the first

several minutes of moderate and heavy exercise, and that only

the PCO 2 changes (actually decreases) during more prolonged

exercise.

7.40

7.45

30

35

40

90

85

95

105

100

110

7.35

pH

P

co

(mmHg) 2

Po

(mmHg) 2

0 10 203040 50

Time (min)

Moderate

Heavy