Page 149 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Effects of High Altitude on Respiration prevail. Stimulation of O 2 chemosensors at
high altitudes also leads to an increase in the
At sea level, the average barometric pressure heart rate and a corresponding increase in car-
(PB) ! 101 kPa (760 mmHg), the O 2 fraction in diac output, thereby increasing the O 2 supply
ambient air (FI O 2 ) is 0.209, and the inspiratory to the tissues.
partial pressure of O 2 (PI O 2 ) ! 21 kPa (! p. 106). High altitude also stimulates erythropoiesis
However, PB decreases with increasing altitude (! p. 88ff.). Prolonged exposure to high alti-
(h, in km): tudes increases the hematocrit levels, al-
PB (at h) = PB (at sea level) ! e -0.127 ! h [5.9] though this is limited by the corresponding
This results in a drop in PI O 2 (! A, column 1), rise in blood viscosity (! pp. 92, 188).
alveolar P O 2 (PA O 2 ) and arterial P O 2 (Pa O 2 ). The Breathing oxygen from pressurized O 2 cyl-
PA O 2 at sea level is about 13 kPa (! A, column inders is necessary for survival at altitudes
2). PA O 2 is an important measure of oxygen above 7000 m, where PI O 2 is almost as high as
supply. If the PA O 2 falls below a critical level (ca. the barometric pressure PB (! A, column 3).
4.7 kPa = 35 mmHg), hypoxia (! p. 130) and The critical PA O 2 level now occurs at an altitude
impairment of cerebral function will occur. of about 12 km with normal ventilation, and at
The critical PA O 2 would be reached at heights of about 14 km with increased ventilation. Mod-
ern long-distance planes fly slightly below this
about 4000 m above sea level during normal
Respiration However, the low Pa O 2 triggers chemosensors altitude to ensure that the passengers can sur-
ventilation (! A, dotted line in column 2).
vive with an oxygen mask in case the cabin
pressure drops unexpectedly.
(! p. 132) that stimulate an increase in total
.
Survival at altitudes above 14 km is not
5 ventilation (V E); this is called O 2 deficiency possible without pressurized chambers or
ventilation (! A, column 4). As a result, larger
volumes of CO 2 are exhaled, and the PA CO 2 and pressurized suits like those used in space
Pa CO 2 decrease (see below). As described by the travel. Otherwise, the body fluids would begin
alveolar gas equation, to boil at altitudes of 20 km or so (! A), where
PB is lower than water vapor pressure at body
PA O 2 ! PI O 2 " [5.10]
PA CO 2
RQ temperature (37 #C).
where RQ is the respiratory quotient
(! pp. 120 and 228), any fall in PA CO 2 will lead Oxygen Toxicity
to a rise in the PA O 2 . O 2 deficiency ventilation
stops the PA O 2 from becoming critical up to alti- Hyperoxia occurs when PI O 2 is above normal
tudes of about 7000 m (altitude gain, ! A). ($ 22 kPa or 165 mmHg) due to an increased
The maximal increase in ventilation (! 3 " O 2 fraction (oxygen therapy) or to an overall
resting rate) during acute O 2 deficiency is rela- pressure increase with a normal O 2 fraction
tively small compared to the increase (! 10 (e.g. in diving, ! p. 134). The degree of O 2 tox-
times the resting rate) during strenuous physi- icity depends on the PI O 2 level (critical: ca.
cal exercise at normal altitudes (! p. 74, C3) 40 kPa or 300 mmHg) and duration of hyper-
because increased ventilation at high altitudes oxia. Lung dysfunction (! p. 118, surfactant
reduces the Pa CO 2 (= hyperventilation, ! p. deficiency) occurs when a PI O 2 of about 70 kPa
108), resulting in the development of respira- (525 mmHg) persists for several days or
tory alkalosis (! p. 144). Central chemosen- 200 kPa (1500 mmHg) for 3–6 hours. Lung dys-
sors (! p. 132) then emit signals to lower the function initially manifests as coughing and
respiratory drive, thereby counteracting the painful breathing. Seizures and unconscious-
signals from O 2 chemosensors to increase the ness occur at PI O 2 levels above 220 kPa
respiratory drive. As the mountain climber (1650 mmHg), e.g., when diving at a depth of
adapts, respiratory alkalosis is compensated about 100 m using pressurized air.
–
for by increased renal excretion of HCO 3 Newborns will go blind if exposed to PIO 2
(! p. 144). This helps return the pH of the levels much greater than 40 kPa (300 mmHg)
blood toward normal, and the O 2 deficiency- for long periods of time (e.g., in an incubator),
136 related increase in respiratory drive can now because the vitreous body then opacifies.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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