Ganong's Review of Medical Physiology, 23rd Edition

618 SECTION VII Respiratory Physiology
Pressure (mm Hg)
FIGURE 36–12 Composition of alveolar air in individuals breathing air (0–6100 m) and 100% O 2 (6100–13,700 m). The minimal
alveolar PO 2 that an unacclimatized subject can tolerate without loss of consciousness is about 35–40 mm Hg. Note that with increasing altitude,
the alveolar PCO 2 drops because of the hyperventilation due to hypoxic stimulation of the carotid and aortic chemoreceptors. The fall in barometric
pressure with increasing altitude is not linear, because air is compressible.
to definitely increase ventilation. As one ascends higher, the
alveolar PO 2 falls less rapidly and the alveolar PCO 2 declines
somewhat because of the hyperventilation. The resulting fall
in arterial PCO 2 produces respiratory alkalosis.
HYPOXIC SYMPTOMS BREATHING AIR
A number of compensatory mechanisms operate over a period
of time to increase altitude tolerance (acclimatization), but in
unacclimatized subjects, mental symptoms such as irritability
appear at about 3700 m. At 5500 m, the hypoxic symptoms are
severe; and at altitudes above 6100 m (20,000 ft), consciousness
is usually lost.
HYPOXIC SYMPTOMS
BREATHING OXYGEN
0
760
720
680
640
600
320
280
240
200
160
120
The total atmospheric pressure becomes the limiting factor in
altitude tolerance when breathing 100% O 2 .
The partial pressure of water vapor in the alveolar air is
constant at 47 mm Hg, and that of CO 2 is normally 40 mm
Hg, so that the lowest barometric pressure at which a normal
alveolar PO 2 of 100 mm Hg is possible is 187 mm Hg, the
pressure at about 10,400 m (34,000 ft). At greater altitudes, the
increased ventilation due to the decline in alveolar PO 2 lowers
the alveolar PCO 2 somewhat, but the maximum alveolar PO 2
80
40
0
Altitude (m)
3000 6000 9000 12,000 15,000 18,000 21,000
Highest permanent
human habitations
(5500 m) Loss of consciousness
if unacclimatized
breathing air
N 2
O 2
CO 2
H 2 O
Top of Mt. Everest
(8854 m)
Alveolar PO2 100 mm Hg
(10,400 m)
Alveolar PO2 40 mm Hg
(13,700 m)
Loss of consciousness
breathing 100% O2 Body fluids boil at
37° C
Breathing air Breathing 100% O 2 Life impossible without
pressurization
that can be attained when breathing 100% O 2 at the ambient
barometric pressure of 100 mm Hg at 13,700 m is about 40
mm Hg. At about 14,000 m, consciousness is lost in spite of
the administration of 100% O 2 . At 19,200 m, the barometric
pressure is 47 mm Hg, and at or below this pressure the body
fluids boil at body temperature. The point is largely academic,
however, because any individual exposed to such a low pressure
would be dead of hypoxia before the bubbles of steam
could cause death.
Of course, an artificial atmosphere can be created around
an individual; in a pressurized suit or cabin supplied with O 2
and a system to remove CO 2 , it is possible to ascend to any
altitude and to live in the vacuum of interplanetary space.
Some delayed effects of high altitude are discussed in Clinical
Box 36–4.
ACCLIMATIZATION
(19,200 m)
Acclimatization to altitude is due to the operation of a variety
of compensatory mechanisms. The respiratory alkalosis produced
by the hyperventilation shifts the oxygen–hemoglobin
dissociation curve to the left, but a concomitant increase in red
blood cell 2,3-BPG tends to decrease the O 2 affinity of hemoglobin.
The net effect is a small increase in P 50 . The decrease
in O 2 affinity makes more O 2 available to the tissues. However,
the value of the increase in P 50 is limited because when the