Human Physiology, 14th edition (2016)

Respiratory Physiology 571

of exercise, because of the time lag required to make proper
cardiovascular adjustments. During this time, therefore, the
muscles metabolize anaerobically, and a “stitch in the side”—
possibly due to hypoxia of the diaphragm—may develop. After
numerous cardiovascular and pulmonary adjustments have
been made, a person may experience a “second wind” when
the muscles are receiving sufficient oxygen for their needs.
Continued heavy exercise can cause a person to reach
the lactate threshold, which is the maximum rate of oxygen
consumption that can be attained before anaerobic metabo-
lism produces a rise in the blood lactate level. This generally
occurs when the exercising person reaches 50% to 70% of the
maximal oxygen uptake. The rise in lactic acid levels is due to
the aerobic limitations of the muscles; it is not due to a mal-
function of the cardiopulmonary system. Indeed, the arterial
oxyhemoglobin saturation remains at 97% and venous blood
draining the muscles contains unused oxygen.
The lactate threshold is higher in endurance-trained ath-
letes than it is in other people. These athletes, because of their
higher cardiac output, have a higher rate of oxygen delivery
to their muscles. Endurance training also increases the skel-
etal muscle content of mitochondria and Krebs cycle enzymes
(chapter 12, section 12.4), enabling the muscles to utilize more
of the oxygen delivered to them by the arterial blood. The
effects of exercise and endurance training on respiratory func-
tion are summarized in table 16.13.

Acclimatization to High Altitude

When a person from a region near sea level moves to a sig-
nificantly higher elevation, several adjustments in respiratory
function are made to compensate for the decreased P^ O 2 at the
higher altitude. These adjustments include changes in ventila-
tion, in hemoglobin affinity for oxygen, and in total hemoglo-
bin concentration.
Reference to table  16.14 indicates that at an altitude
of 7,500 feet, for example, the P^ O 2 of arterial blood is 69 to
74 mmHg (compared to 90 to 95 mmHg at sea level). This table
also indicates that the percent oxyhemoglobin saturation at this

altitude is between 92% and 93%, compared to about 97% at

sea level. The amount of oxygen attached to hemoglobin, and

thus the total oxygen content of blood, is therefore decreased. In

addition, the rate at which oxygen can be delivered to the cells

(by the plasma-derived tissue fluid) after it dissociates from oxy-

hemoglobin is reduced at the higher altitude. This is because the

maximum concentration of oxygen that can be dissolved in the

plasma decreases in a linear fashion with the fall in P^ O 2. People

may thus experience rapid fatigue even at more moderate eleva-

tions (for example, 5,000 to 6,000 feet), at which the oxyhe-

moglobin saturation is only slightly decreased. Compensations

made by the respiratory system gradually reduce the amount of

fatigue caused by a given amount of exertion at high altitudes.

Table 16.13 | Changes in Respiratory Function During Exercise

Variable Change Comments

Ventilation Increased In moderate exercise, ventilation is matched to increased metabolic rate. Mechanisms

responsible for increased ventilation are not well understood.

Blood gases No change Blood gas measurements during light and moderate exercise show little change

because ventilation is increased to match increased muscle oxygen consumption

and carbon dioxide production.

Oxygen delivery to muscles Increased Although the total oxygen content and PO 2 do not increase during exercise, there is an

increased rate of blood flow to the exercising muscles.

Oxygen extraction by muscles Increased Increased oxygen consumption lowers the tissue PO 2 and lowers the affinity of

hemoglobin for oxygen (due to the effect of increased temperature). More oxygen,

as a result, is unloaded so that venous blood contains a lower oxyhemoglobin

saturation than at rest. This effect is enhanced by endurance training.

Table 16.14 | Blood Gas Measurements

at Different Altitudes

Altitude

Arterial PO 2

(mmHg)

Percent

Oxyhemoglobin

Saturation

Arterial

PCO 2

(mmHg)

Sea level 90–95 97% 40

1,524 m

(5,000 ft)

75–81 95% 32–33

2,286 m

(7,500 ft)

69–74 92%–93% 31–33

4,572 m

(15,000 ft)

48–53 86% 25

6,096 m

(20,000 ft)

37–45 76% 20

7,620 m

(25,000 ft)

32–39 68% 13

8,848 m

(29,029 ft)

26–33 58% 9.5–13.8

Source: From P. H. Hackett et al.,“High Altitude Medicine” in Management

of Wilderness and Environmental Emergencies, 2d ed., edited by Paul S.

Auerbach and Edward C. Geehr. Copyright © 1989 Mosby-Yearbook.

Reprinted by permission.