{"id":424,"date":"2019-06-24T14:04:13","date_gmt":"2019-06-24T18:04:13","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/8-8-human-responses-to-high-altitude\/"},"modified":"2022-01-19T15:39:45","modified_gmt":"2022-01-19T20:39:45","slug":"8-8-human-responses-to-high-altitude","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/8-8-human-responses-to-high-altitude\/","title":{"raw":"6.6\u00a0Human Responses to High Altitude","rendered":"6.6\u00a0Human Responses to High Altitude"},"content":{"raw":"Created by: CK-12\/Adapted by Christine Miller\n\n[caption id=\"attachment_419\" align=\"aligncenter\" width=\"400\"]\"Humans Figure 6.6.1 Machu Picchu in the Peruvian Andes.<\/em>[\/caption]\n\n
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High and Hypoxic<\/h1>\n<\/div>\nThis mountain scene of Machu Picchu in the Peruvian Andes is a sight to behold. Lurking behind the beauty of this and some other mountain ranges, however, is a potentially deadly threat to the human organism: high-altitude hypoxia. [pb_glossary id=\"2008\"]Hypoxia[\/pb_glossary]<\/strong> is literally a lack of oxygen. It occurs to varying degrees at altitudes higher than about 2,500 metres above sea level. Yet despite the high altitude of the location shown in Figure 6.6.1, it is very evident that humans have been thriving in this environment for long periods of time; in fact, Machu Picchu was most likely built in the mid 1400s.\u00a0 Modern day peoples live in high altitude locations all over earth where hypoxia may occur, including\u00a0 the Himalaya Mountains in Asia, the Ethiopian Highlands in Africa, and the Rocky Mountains in North America.\n
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Why Hypoxia Occurs at High Altitudes<\/h1>\n<\/div>\nAlthough the percentage of oxygen in the atmosphere is the same at high altitudes as it is at sea level, the atmosphere is less dense at high altitudes. This means that the molecules of oxygen (and other gases) in the air are more spread out, so a given volume of air contains fewer oxygen molecules. This results in lower air pressure at high altitude. Air pressure decreases exponentially as altitude increases, as shown in the graph below (Figure 6.6.2).\n\n[caption id=\"attachment_419\" align=\"aligncenter\" width=\"769\"]\"Atmospheric Figure 6.6.2 As altitude increases, atmospheric pressure decreases, which means there are fewer molecules of oxygen in a single breath at high elevations than a single breath at lower elevations.<\/em>[\/caption]\n\nAt sea level, air pressure is about 100 kPa. At this air pressure, the air is dense and oxygen passes easily from the air in the lungs through cell membranes into the bloodstream. This is because concentration affects diffusion \u2014 the higher the concentration of oxygen in the air we breath, the more it will diffuse into our blood.\u00a0 It is likely we evolved at or near sea level altitudes, so it is not surprising that the human body generally performs best at this altitude. However, as air pressure decreases at high altitudes, it becomes more difficult for adequate oxygen to pass into the bloodstream, and blood levels of oxygen start to fall.\n\nAt 2,500 metres above sea level, air pressure is only about 75 per cent of that at sea level, and at five thousand metres, air pressure is only about 50 per cent of the sea level value. The latter altitude is about the altitude of the Mount Everest Base Camp and of the highest permanent human settlement (La Rinconada in Peru, pictured in Figure 6.6.3). Altitudes above 2,500 metres generally require acclimatization or adaptation to prevent illness from hypoxia. Above 7,500 metres, serious symptoms of hypoxia are likely to develop. Altitudes above eight thousand metres are in the \u201cdeath zone.\u201d This is the zone where hypoxia becomes too great to sustain human life. The summit of Everest, with an altitude of 8,848 metres, is well within the death zone. Mountain climbers can survive there only by taking in extra oxygen from oxygen tanks and not staying at the summit very long.<\/span>\n\n[caption id=\"attachment_419\" align=\"aligncenter\" width=\"400\"]\"La Figure 6.6.3 La Rincondada, Peru \u2014 the highest permanent human habitation.<\/em>[\/caption]\n\n
\n\nPhysiological Effects of Hypoxia<\/span>\n\n<\/div>\nWhen a lowlander first goes to an altitude above 2,500 metres, the person\u2019s blood oxygen level starts to fall. The immediate responses of the body to hypoxia are not very efficient, and they place additional stress on the body. The main changes are an increase in the breathing rate (hyperventilation) and an elevation of the heart rate. These rates may be as much as double<\/em>\u00a0their normal levels, and they may persist at high levels, even during rest. While these changes increase oxygen intake in the short term, they also place more stress on the body. For example, hyperventilation causes respiratory alkalosis, in which carbon dioxide levels in the blood become too low. The increased heart rate places stress on the cardiovascular system and may be especially dangerous for someone with an underlying heart problem.\n\nThe first symptoms of hypoxia the lowlander is likely to notice is becoming tired and out of breath when performing physical tasks. Appetite is also likely to decline, as nonessential body functions are shut down at the expense of maintaining rapid breathing and heart rates. Other symptoms are also likely to develop, such as headache, dizziness, distorted vision, ringing in the ears, difficulty concentrating, insomnia, nausea, and vomiting. These are all symptoms of\u00a0[pb_glossary id=\"1496\"]high altitude sickness[\/pb_glossary]<\/strong>.\n\nMore serious symptoms may also develop at high altitudes.\u00a0Fluid collects in the lungs (high altitude pulmonary edema, or HAPE<\/a>) and in the brain (high altitude cerebral edema, or HACE<\/a>). HACE may result in permanent brain damage, and both HAPE and HACE can be fatal. The higher the altitude, the greater the likelihood of these serious high altitude disorders occurring, and the greater the risk of death.\n
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Acclimatization to High Altitude<\/h1>\n<\/div>\n\n[caption id=\"attachment_419\" align=\"alignright\" width=\"521\"]\"Athletic Figure 6.6.4 Endurance athletes may train at high elevations to build up their red blood cell count and muscle capillaries and then compete at lower elevations with an advantage.<\/em>[\/caption]\n\nIf a lowlander stays at high altitude for several days, the body starts to respond in ways that are less stressful. These responses are the result of acclimatization to high altitude. Additional red blood cells are produced and the tiniest blood vessels, called capillaries, become more numerous in muscle tissues. The lungs also increase slightly in size, as does the right ventricle of the heart, which is the heart chamber that pumps blood to the lungs. All of these changes make the processes of taking in oxygen and transporting it to cells more efficient.\n\nIt might occur to you that these changes with acclimatization would improve fitness and performance in athletes, and you would be right. The same changes that help the body cope with high altitude increase fitness and performance at lower altitudes. That\u2019s why athletes often travel to high altitudes to train, and then compete at lower altitudes. Figure 6.6.4 shows Olympic athletes training for long distance running at the Swiss Olympic Training Base in St. Moritz, located in the Swiss Alps.\n\nFull acclimatization to high altitude generally takes several weeks. The higher the altitude, the longer it takes. Even when acclimatization is successful and symptoms of high altitude sickness mostly abate, the lowlander may not be able to attain the same level of physical or mental performance as is possible at lower altitudes. When an altitude acclimatized individual returns to sea level, the changes that occurred at high altitude are no longer needed. The body reverts to the original, pre-high-altitude state in a matter of weeks.\n
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Genetic Adaptations to High Altitude<\/h1>\n<\/div>\nWell over 100 million people worldwide are estimated to live at altitudes higher than 2,500 metres above sea level. In Table 6.6.1, you can see how these people are distributed in the highest altitude regions around the globe.\n\nTable 6.6.1<\/strong>\n\nHuman Populations Residing in High Altitude Regions<\/em>\n\n\n\n\n\n\n\n\n
Human Populations Residing in High Altitude Regions<\/strong><\/td>\n<\/tr>\n
High Altitude Region<\/strong><\/td>\nHuman Population<\/strong><\/td>\n<\/tr>\n
Himalaya-Hindukush-Pamir Ranges, Tibetan Plateau (Asia)<\/td>\n78,000,000<\/td>\n<\/tr>\n
Andes Mountains (South America)<\/td>\n35,000,000<\/td>\n<\/tr>\n
Ethiopian Highlands (Africa)<\/td>\n13,000,000<\/td>\n<\/tr>\n
Rocky Mountains (North America)<\/td>\n300,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nSome Indigenous populations of Tibet, Peru, and Ethiopia have been living above 2,500 metres for hundreds of generations and have evolved genetic adaptations that protect them from high altitude hypoxia. In these populations, [pb_glossary id=\"1547\"]natural selection[\/pb_glossary] has brought about irreversible, genetically-controlled changes that adapt them to high altitude conditions. As a result, they can live permanently at high altitudes without any, or with only minor, ill effects \u2014 even though they are constantly exposed to a level of oxygen that would cause high altitude sickness in most other people. Interestingly, different adaptations evolved in different regions in response to the same stress.\n

High Altitude Adaptations in Tibetan Highlanders<\/h2>\nHighland populations in Tibet, such as the famous Sherpas who serve as Himalaya Mountain guides (see Figure 6.6.5), have lived at high altitudes for only about three thousand years. Their adaptations to high altitude include an increase in the rate of breathing even at rest without alkalosis occurring, and an expansion in the width of the blood vessels (both capillaries and arteries) that carry oxygenated blood to the cells. These changes allow them to carry more oxygen to their muscles and have a higher capacity for exercise at high altitude. Their adaptations to high altitude occurred very rapidly in evolutionary terms and are considered to be the most rapid process of phenotypically observable evolution in humans.\n\n[caption id=\"attachment_419\" align=\"aligncenter\" width=\"413\"]\"\" Figure 6.6.5 The flushed skin of this Tibetan Sherpa guide is due to the increased arterial blood flow that is a genetic adaptation to high altitude hypoxia in Tibetan highlanders.<\/em>[\/caption]\n\n
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High Altitude Adaptations in Andean Highlanders<\/span><\/h2>\n<\/div>\nAndean highlanders, such as Quechua Native Americans (see Figure 6.6.6), have been living at high altitudes for about 11 thousand years. Their genetic adaptations to high altitude are different than the Tibetan adaptations. They include greater red blood cell volume and increased concentration of hemoglobin, the oxygen-carrying protein that is the main component of red blood cells. These changes allow somewhat higher levels of oxygen to circulate in the blood without increasing the rate of breathing. Compared with other long-term residents at high altitudes, Andean highlanders are the least adapted and most likely to experience high altitude sickness.\n\n\n\n\n
\"\"<\/td>\n\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
Figure 6.6.6 Quechua Native Americans in the Peruvian Andes.<\/span><\/em><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

High Altitude Adaptations in Ethiopian Highlanders<\/h3>\n[caption id=\"attachment_419\" align=\"alignleft\" width=\"333\"]\"\" Figure 6.6.7 Ethiopian Highlands in East Africa.<\/em>[\/caption]\n\nThe Ethiopian Highlands (Figure 6.6.7) are high enough to have brought about genetic adaptations in long-term residents. Populations of Ethiopian Highlanders have lived above 2,500 metres for at least five thousand years, and above two thousand metres for as long as 70 thousand years. Many Ethiopian Highlanders today live at altitudes greater than 3,000 metres. However, Ethiopian Highland populations do not appear to have evolved the adaptations that characterize either Tibetan highlanders or Andean highlanders. They do not exhibit the hemoglobin changes or vascular changes of these other highland populations, but they do have greater arterial blood oxygen saturation. Research on Ethiopian adaptations to high altitude has just begun and is still very limited, but they appear to have a unique pattern of adaptation.\n\n \n
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6.6 Summary<\/span><\/h1>\n<\/header>\n
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