Travel Reference
In-Depth Information
Acclimatization can best be described as a series of integrated changes by which the
partial pressure of oxygen in the blood reaching the tissues is brought closer to that in the
ambient air. It is a gradual process, taking days to weeks, but a well-acclimatized person
can tolerate altitudes that would soon incapacitate and might kill a person newly arrived
from sea level. It is also a remarkable process that allows many persons who are hypoxic
at sea level as the result of lung or heart disease to lead nearly normal lives.
The most important changes in acclimatization are those that occur upon first arrival at
altitude:
◆
Increased respiratory volume
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Increased cardiac output
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Elevation of pulmonary artery pressure
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Apparent increase in red blood cells due to a shift of fluid from blood to tissues and
loss of fluid in the urine
In addition, complex changes in the way cells use oxygen, hormonal changes that con-
trolelectrolytemigration,changesinurineoutput,andredistributionofbloodflowtomore
critical parts of the body all promote normal function at low oxygen pressures.
Increased Respiratory Volume (Ventilation)
An increase in the depth and, to a lesser extent, the rate of respiration—increased vent-
ilation—brings more outside air into the alveoli, increasing the oxygen available to diffuse
into blood. The increase is most obvious during exercise. Those who have just arrived at
even moderate altitude may experience unusual shortness of breath during only moderate
exertion. Ventilation is the function that limits exercise at extreme altitudes.
Increased Pulmonary Artery Pressure
Many conditions that reduce the oxygen pressure in the lungs, whether at altitude or at
sealevel,increasethebloodpressureinthepulmonaryarteries.Theelevatedpressuretends
to open more capillaries in all parts of the lung (many of which are closed during quiet
respiration at sea level) to maximize the capacity of the pulmonary circulation to absorb
oxygen.Oneexplanationofferedforhigh-altitudepulmonaryedemaisthatincreasedarter-
ial pressure, transmitted directly to the capillaries, forces fluid through thin capillary walls
into the alveoli.
Lack of tissue oxygen due to anemia or carbon monoxide poisoning (carbon monoxide
reacts with hemoglobin to prevent it from carrying oxygen) does not increase pulmonary
arterial pressure because the alveolar oxygen is normal. On the other hand, sleep apnea,
which can lower alveolar oxygen several times each minute, can increase pulmonary arter-
ial pressure permanently and is thought to increase systemic blood pressure, too.
Increased Cardiac Output
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