Hi, reg77-ga!
Your question is an interesting one to me, since I live in an area
filled with Olympic athletes who train at high altitude to increase
their exercise efficiency. I also had quite a time adjusting to the
high altitude when I moved here ten years ago, so I have already done
quite a bit of research into altitude adaptation.
The most basic answer to your question is that human blood must
manufacture more hemoglobin in order to utilize available oxygen at
higher altitude. Total blood volume must also increase. Until that
occurs, people often feel breathless and dizzy as they are not getting
enough oxygen to their tissues. It is not unusual for aging skiiers to
die on the slopes of a heart attack, when flying out here from low
altitude for a ski vacation. Therefore, the importance of adjusting to
high altitude in increments, and exercising slowly at first, cannot be
stressed enough.
The following information is excerpted from the rather extensive
article' "High Altitude and its Effects on Exercise Performance," by
Dan Graetzer. It should explain the answer to your question in
thorough detail.
"The human body requires a continuous supply of oxygen to the
tissues to maintain the process of metabolism (the use of
substrate for energy to maintain life-sustaining biological
processes). The source of this oxygen is the ambient air where
the percentage of oxygen remains fixed at 20.93% regardless of
altitude. Ascent to a higher altitude causes a reduction in
barometric pressure which induces a corresponding decrease in
the partial pressure of oxygen of the inhaled air. For
example, with ascent from sea level to the top of the tram at
Snowbird's 11,000 foot Hidden Peak, the average barometric
pressure decreases from about 760 mm Hg to about 510 mm Hg.
This reduces the partial pressure of the inspired air from
about 149 mm Hg to about 97 mm Hg. Partial pressure of the
inspired air after it has been inhaled into the lungs and is
fully saturated with water vapor is calculated as barometric
pressure minus 47 times the percentage of oxygen in the
ambient air:
(Snowbird PIO2 = ((510 - 47) x .2093) = 96.9 mm Hg.
This oxygen enters the body through the lungs where it binds
reversible with hemoglobin in the bloodstream for transport to
the tissues. A reduced partial pressure of oxygen will impair
the oxygenation of blood flowing through the lungs. A
bloodstream with a reduced oxygen saturation will consequently
deliver diminished oxygen supply to the working muscles.
At the muscle tissue level, oxygen is released from the blood
and enters the cells of the working muscles to sustain aerobic
metabolism. The preferred fuel for exercise at altitude
appears to be fat due to a dramatic decrease in carbohydrate
metabolism. This complex shift in substrate utilization is not
well understood but may be due to the fact that a reduced
oxygen supply already causes a higher lactic acid level in the
muscles and bloodstream and carbohydrate consumption must be
curtailed.
Lactate is only produced during carbohydrate (not fat)
breakdown. The end result of these occurrences is reduced
maximal aerobic power, diminished endurance capacity, and
earlier muscular fatigue during your high altitude ski
vacation.
Fortunately, there are several complex, physiological,
interactions that work to minimize the effects of a reduced
oxygen delivery to the tissues. Many of these adaptations
occur quite early after high altitude exposure and include
shifts in pul monary ventilation, the cardiovascular system,
and the cellular composition of the blood.
Ventilation rate (total amount of air moving in and out of the
lungs) is stimulated at high elevations by an increase in
breath frequency. This serves to raise oxygen availability to
the alveoli in the lungs (site of oxygen extraction from the
pulmonary system into the bloodstream).
Unfortunately, hyperventilation also blows off excess carbon
dioxide from the body which has the potential to disrupt
acid-base balance of the tissues and contribute to altitude
sickness. Considerable body water is also lost with high
ventilatory rates leading to a relative state of dehydration.
Airplane flights to higher altitudes also strongly contribute
to dehydration because the relative humidity of airplane
cabins and mountainous regions are generally quite low. A low
humidity environment continually draws precious moisture from
the body which must be replaced by fluid consumption.
Altitude also stimulates an increase in heart rate and cardiac
output (total amount of blood pumped by the heart) to increase
blood circulation by the muscles to unload oxygen and pick up
carbon dioxide and back to the alveoli to reverse these
exchanges. This serves to compensate for the blood's reduced
oxygen saturation but also provides more stress to the heart
which may effect persons predisposed to heart disease.
**The composition of the blood changes after about 2 weeks of
altitude exposure by producing more red blood cells and
hemoglobin (the iron-protein compound that transports oxygen).
Bone marrow stimulation to increase hematocrit (percentage of
red cells in the blood) in addition to an increase in plasma
volume serves to increase total blood volume. The benefits of
blood adaptation in the weeks following exposure includes
reducing the cardiac output required for oxygen delivery
during rest and submaximal exercise, increasing maximal oxygen
transport during strenuous exertion, and providing a larger
fluid reserve for sweating."**
You may read the entire article, which is quite fascinating, at
http://www.sumeria.net/oxy/altitude.html
Hope this information provides the information you need!
umiat-ga
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