|MadSci Network: Medicine|
|MadSci Network: Medicine|
The basic problem on moving to high altitudes is the transfer of oxygen from the air to the tissues, where it is used in the production of energy. This transfer is accomplished across a number of potential gradients. These gradients are kind of like water falling down a waterfall, and the steps are often represented in a graph that shows the changes in potential as an "oxygen cascade."
Looking at the graph, you will see that as you move from the air (far left), into the lungs, and the arteries, the potential for the oxygen is continually decreasing. This drop in potential is the physical element that drives the oxygen into the tissues. Near the far right of the graph, the potentials are all dashed. This is to indicate that the exchange of oxygen with the tissues takes place at this stage, as the blood passes through the capillaries. The last column is the blood in the veins
You'll also note that there are several cascades drawn, each one represents the change in the oxygen cascade as you move to progressively higher altitudes. The top is the cascade at sea level, the second is at 4540 meters (about 14,900 feet, a bit higher than Mt. Rainier), and the lowest is 6700 meters (almost 22,000 feet, which is near the height of Huascaran, the highest point in Peru). You'll see that the drop in pressure between compartments is decreased by increasing altitude.
Of course, since the air pressure is much lower at higher altitudes, there is less of a potential driving oxygen to the tissues. The body uses many mechanisms to compensate for this decrease in potential, and those mechanisms are called acclimation.
Among the important changes that are a part of high-altitude acclimation are increases in the total volume of air inspired (first stage of the oxygen cascade) and an increase in the oxygen-carrying capacity of the blood (second stage of the oxygen cascade).
If one goes to a high altitude faster than the body is able to adjust, then various symptoms set in. Everyone can expect to experience hyperventilation, tachycardia (speeding heart) and palpitations (feeling the heart beat), and troubles with sleeping and digestion.
In about half of the people who go above 4240 m, more serious problems
begin which signal the onset of acute mountain sickness. These include
headache, insomnia, nausea, decreased urine, and incoordination. These
symptoms can occasionally progress into HAPE (high altitude pulmonary
edema) or HACE (high altitude cerebral edema) -- both life-threatening
conditions. It appears that these symptoms are all secondary to changes
in the distribution of water in the body.
The rate at which people acclimate varies widely. It depends on obvious things like the altitude at which they were raised and whether they have previously been at high altitudes. Much subtler variables can also impact acclimitization: some evidence has been presented that miners going to work at high altitudes are less impacted by problems acclimatizing because they simply wish to get on with their work, and are able to accept the discomfort as temporary!
In addition to these factors, one also has to consider the fundamental variability that can be observed in biology. Since there are several steps in the oxygen cascade, all of which show some changes during acclimation, one would expect to see a wide variation in the responses of individuals, even given similar previous experiences with high altitude.
My main source for this was Man at High Altitude by D Heath and DR Williams, Churchill Livingstone, New York, 1981.
Mail questions to Steve Helms Tillery.
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