763
CHAPTER NINETEEN
Respiratory System
E
very year, about 100,000 mountain climb-
ers experience varying degrees of altitude
sickness, because at high elevations, the
proportion of oxygen in air remains the same
(about 21%), but the P
O
2
decreases. When a per-
son ascends rapidly, oxygen diF
uses more slowly
from the alveoli into the blood, and the hemo-
globin becomes less saturated with oxygen. In
some individuals, the body’s eF
orts to get more
oxygen—increased breathing and heart rate and
enhanced red blood cell and hemoglobin pro-
duction—cannot keep pace with the plummet-
ing oxygen supply.
Severe altitude sickness includes a condition
called high-altitude pulmonary edema (HAPE).
Symptoms are sudden severe headache, nausea
and vomiting, rapid heart rate and breathing, and
a cyanotic (blue) cast to the skin, often ±
rst appar-
ent under the ±
ngernails.
The hypoxia associated with high altitude
can cause vasoconstriction of pulmonary blood
vessels. In some persons, this shunts blood under
high pressure through less constricted vessels in
the pulmonary circuit, raising capillary pressure
and filtering fluid from the blood vessels into
the alveoli. Persons with severe HAPE commonly
develop high-altitude cerebral edema (HACE).
HAPE is treated by giving oxygen and com-
ing down from the mountain. Delay may prove
fatal. Exertion may worsen the symptoms, and
victims often need to be carried. Some prescrip-
tion vasodilators, such as nifedipine, may help
reduce the pulmonary hypertension, but they can
be dangerous without proper medical attention.
Mountain climbing is an extreme activity
that endangers the respiratory system. Regularly
exercising at moderately high altitude, however,
can strengthen the system. Analysis of the results
of 1,460 international football competitions, cov-
ering 100 years, revealed that teams accustomed
to altitudes greater than 2,500 meters (about 1.5
miles) above sea level scored signi±
cantly more
touchdowns, at any altitude, than did teams that
practiced at much lower altitudes. In response to
the study, the ²ederation of International ²ootball
Associations banned matches above this level
because the teams that trained at higher eleva-
tions had an unfair advantage. Distance runners,
too, often train at high altitudes because of ben-
cial eF
ects on ±
tness.
19.5
CLINICAL APPLICATION
Effects of High Altitude
it possible for the respiratory system to safely maintain CO
2
levels, and thereby the pH, of the internal environment.
PRACTICE
40
How is oxygen transported from the lungs to body cells?
41
What factors aF
ect the release of oxygen from oxyhemoglobin?
oxygen molecule. As oxygen dissolves in blood, it rapidly
combines with hemoglobin, forming a new compound called
oxyhemoglobin
(ok
sı˘-he
mo-glo
bin). Each hemoglobin
molecule can bind up to four oxygen molecules.
The P
O
2
determines the amount of oxygen that hemoglo-
bin binds. The greater the P
O
2
, the more oxygen binds until
the hemoglobin molecules are saturated
(f
g. 19.36)
. At nor-
mal arterial P
O
2
(95 mm Hg), hemoglobin is essentially com-
pletely saturated.
The chemical bonds between oxygen and hemoglobin
molecules are relatively unstable, and as the P
O
2
decreases,
oxyhemoglobin releases oxygen molecules (F
g. 19.36). This
happens in tissues in which cells have used oxygen in respi-
ration. The free oxygen diffuses from the blood into nearby
cells, as
f gure 19.37
shows.
Increasing blood concentration of carbon dioxide (P
CO
2
),
acidity, and temperature all increase the amount of oxygen that
oxyhemoglobin releases
(f gs. 19.38, 19.39,
and
19.40)
. These
infl uences explain why more oxygen is released from the blood
to the skeletal muscles during exercise. The increased muscu-
lar activity accompanied by increased oxygen use increases
the P
CO
2
, decreases the pH, and raises the local temperature. At
the same time, less-active cells receive less oxygen.
As described earlier, respiratory control under most cir-
cumstances is responding to plasma P
CO
2
and pH, not P
O
2
,
despite the central role of oxygen in cellular metabolism.
Notice, however, in F gures 19.36 and 19.37, that the deoxy-
genated systemic venous blood retains 75% of the oxygen it
had when it was fully oxygenated. This safety margin makes
P
O
2
(mm Hg)
10
20
30
40
50
60
70
80
90
100
Oxyhemoglobin dissociation at 38
°
C
% saturation of hemoglobin
10
04
0
5
0
6
0
7
0
9
0
80
100 110 120 130 140
20
30
FIGURE 19.36
Hemoglobin is completely saturated at normal
systemic arterial P
O
2
but readily releases oxygen at the P
O
2
of the body
tissues.
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