DISORDERS CAUSED BY ALTITUDE - Medicine for Mountaineering and Other Wilderness Activities - page 341

As stated, during the first few days at high altitude the volume of blood pumped by the
heart at rest or at any exercise level is higher than at sea level, which increases the amount
of oxygen delivered to the tissues. However, after seven to ten days the cardiac output be-
comes less than at sea level, and more time is required for any specific amount of work,
whetherbriskexercisesuchasrunningormoreprolongedactivitiessuchascarryingaload.
With a longer stay at altitude, cardiac output rises again and, at extreme altitude, is above
sea level values. Again the ability of the heart to pump blood does not limit work at ex-
treme altitude, even during heavy exercise. The ability—or inability—to move air restricts
work at great heights.
Increased Number of Red Blood Cells
Shortly after arrival at high altitude, an apparent increase in the concentration of red
cells in the blood results from movement of water out of the blood into the tissues.
However,hypoxiastimulatesreleaseofthehormoneerythropoietin,whichisformedinthe
lungs and kidneys. This hormone is a powerful bone marrow stimulant, and within the first
few days after arrival red blood cell production actually increases. The blood eventually
may contain many more cells than at sea level. This increase in red cells, measured as the
percentage of whole blood volume occupied by cells (hematocrit), is the best known and
historically was the earliest described change in acclimatization, although it is not the most
important.
The increased number of red cells enables blood to carry more oxygen, but this in-
creased capacity may be offset by the increase in blood viscosity. A serious danger of clot
formation in the veins (thrombophlebitis) appears when the hematocrit rises above 60 per-
cent. (Normal at sea level in males is 47 ± 5 percent; in females 42 ± 5 percent.)
Changes in Oxygen-Carrying Capacity
Red blood cells contain the enzyme 2, 3-diphosphoglycerate (DPG), which facilitates
the release of oxygen from hemoglobin to the tissues. The concentration of DPG in the
bloodincreasesathigheraltitudesandallowsreleaseofalargervolumeoftransportedoxy-
gen for a smaller drop in oxygen pressure, at least below 20,000 feet (6100 m). At higher
elevations, the increased alkalinity caused by loss of carbon dioxide helps blood take on
oxygen in the lungs.
Changes in Body Tissues
Acclimatization by long residence at altitude causes more subtle changes that enable
near normal function by the oxygen-consuming tissues, particularly muscle, despite low
oxygen pressures. These changes include the following:
◆ An increase in the number of capillaries within the muscle
◆ An increase in myoglobin, the intramuscular oxygen-carrying protein