f blood in the circulatory system. The sympathetic nervous system also acts to stimulate vasodilation within the skeletal muscle. At the level of the capillaries the normally closed precapillary sphincters open allowing a large flow of blood through the muscles. In turn the cardiac output increases both in terms of heart rate and stroke volume. The stroke volume, however, does not substantially increase in the non-athlete (Langley, et.al., 1980). This demonstrates an obvious benefit of regular exercise and physical conditioning particularly for an individual who will be exposed to high altitudes. The heart rate is increased by the action of the adrenal medulla which releases catecholamines. These catecholamines work directly on the myocardium to strengthen contraction. Another compensation mechanism is the release of renin by the kidneys. Renin leads to the production of angiotensin which serves to increase blood pressure (Langley, Telford, and Christensen, 1980). This helps to force more blood into capillaries. All of these changes are a regular and normal response of the body to external stressors. The question involved with altitude changes becomes what happens when the normal responses can no longer meet the oxygen demand from the cells? ACUTE MOUNTAIN SICKNESS One possibility is that Acute Mountain Sickness (AMS) may occur. AMS is common at high altitudes. At elevations over 10,000 feet, 75% of people will have mild symptoms (Princeton, 1995). The occurrence of AMS is dependent upon the elevation, the rate of ascent to that elevation, and individual susceptibility. Acute Mountain Sickness is labeled as mild, moderate, or severe dependent on the presenting symptoms. Many people will experience mild AMS during the process of acclimatization to a higher altitude. In this case symptoms of AMS would usually start 12-24 hours after arrival at a higher altitude and begin to decrease in severity about the third day. ...
