Effects of high altitude on humans

The effects of high altitude on humans are mostly the consequences of reduced partial pressure of oxygen in the atmosphere. The medical problems that are direct consequence of high altitude are caused by the low inspired partial pressure of oxygen, which is caused by the reduced atmospheric pressure, and the constant gas fraction of oxygen in atmospheric air over the range in which humans can survive. The other major effect of altitude is due to lower ambient temperature.
The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above as the death zone, where it is generally believed that no human body can acclimatize. At extreme altitudes, the ambient pressure can drop below the vapor pressure of water at body temperature–at such altitudes even pure oxygen at ambient pressure cannot support human life, and a pressure suit is necessary. A rapid depressurisation to the low pressures of high altitudes can trigger altitude decompression sickness.
The physiological responses to high altitude include hyperventilation, polycythemia, increased capillary density in muscle and hypoxic pulmonary vasoconstriction–increased intracellular oxidative enzymes. There are a range of responses to hypoxia at the cellular level, shown by discovery of hypoxia-inducible factors, which determine the general responses of the body to oxygen deprivation. Physiological functions at high altitude are not normal and evidence also shows impairment of neuropsychological function, which has been implicated in mountaineering and aviation accidents. Methods of mitigating the effects of the high altitude environment include oxygen enrichment of breathing air and/or an increase of pressure in an enclosed environment. Other effects of high altitude include frostbite, hypothermia, sunburn, and dehydration.
Tibetans, Andeans, and Amharas are three groups which are relatively well adapted to high altitude, but display noticeably different phenotypes.

Pressure effects as a function of altitude

The human body can perform best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars. The concentration of oxygen in sea-level air is 20.9%, so the partial pressure of O₂ is. In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells.
Atmospheric pressure decreases following the Barometric formula with altitude while the O₂ fraction remains constant to about, so pO₂ decreases with altitude as well. It is about half of its sea-level value at, the altitude of the Everest Base Camp, and only a third at, the summit of Mount Everest. When pO₂ drops, the body responds with altitude acclimatization.
The International Society for Mountain Medicine recognizes three altitude regions which reflect the lowered amount of oxygen in the atmosphere:

High altitude =
Very high altitude =
Extreme altitude = above

Travel to each of these altitude regions can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high-altitude pulmonary edema and high-altitude cerebral edema. The higher the altitude, the greater the risk. Expedition doctors commonly stock a supply of dexamethasone, to treat these conditions on site. Research also indicates elevated risk of permanent brain damage in people climbing to above.
People who develop acute mountain sickness can sometimes be identified before the onset of symptoms by changes in fluid balance hormones regulating salt and water metabolism. People who are predisposed to develop high-altitude pulmonary edema may present a reduction in urine production before respiratory symptoms become apparent.
Humans have survived for two years at, 475 millibars of atmospheric pressure), which is the highest recorded permanently tolerable altitude; the highest permanent settlement known, La Rinconada, is at.
At altitudes above, 383 millibars of atmospheric pressure), sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.

Death zone

The death zone in mountaineering. All 14 summits in the death zone above 8000 m, called eight-thousanders, are located in the Himalaya and Karakoram mountain ranges.
Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly by loss of vital functions or indirectly through wrong decisions made under stress or physical weakening leading to accidents. In the death zone, the human body cannot acclimatize. An extended stay in the death zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness, and, ultimately, death.
File:K2 2006b.jpg|thumb|The summit of K2, the second highest mountain on Earth, is in the death zone.
At an altitude of, the atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. This altitude is known as the Armstrong limit. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds.
Even below the Armstrong limit, an abrupt decrease in atmospheric pressure can cause venous gas bubbles and decompression sickness. A sudden change from sea-level pressure to pressures as low as those at can cause altitude-induced decompression sickness.

Acclimatization

The human body can adapt to high altitude through both immediate and long-term acclimatization. At high altitude, in the short term, the lack of oxygen is sensed by the carotid bodies, which causes an increase in the breathing depth and rate. However, hyperpnea also causes the adverse effect of respiratory alkalosis, inhibiting the respiratory center from enhancing the respiratory rate as much as would be required. Inability to increase the breathing rate can be caused by inadequate carotid body response or pulmonary or renal disease.
In addition, at high altitude, the heart beats faster; the stroke volume is slightly decreased; and non-essential bodily functions are suppressed, resulting in a decline in food digestion efficiency.
Full acclimatization requires days or even weeks. Gradually, the body compensates for the respiratory alkalosis by renal excretion of bicarbonate, allowing adequate respiration to provide oxygen without risking alkalosis. It takes about four days at any given altitude and can be enhanced by drugs such as acetazolamide. Eventually, the body undergoes physiological changes such as lower lactate production, decreased plasma volume, increased hematocrit, increased RBC mass, a higher concentration of capillaries in skeletal muscle tissue, increased myoglobin, increased mitochondria, increased aerobic enzyme concentration, increase in 2,3-BPG, hypoxic pulmonary vasoconstriction, and right ventricular hypertrophy. Pulmonary artery pressure increases in an effort to oxygenate more blood.
Full hematological adaptation to high altitude is achieved when the increase of red blood cells reaches a plateau and stops. The length of full hematological adaptation can be approximated by multiplying the altitude in kilometres by 11.4 days. For example, to adapt to of altitude would require 45.6 days. The upper altitude limit of this linear relationship has not been fully established.
Even when acclimatized, prolonged exposure to high altitude can interfere with pregnancy and cause intrauterine growth restriction or pre-eclampsia. High altitude causes decreased blood flow to the placenta, even in acclimatized women, which interferes with fetal growth. Consequently, children born at high-altitudes are found to be born shorter on average than children born at sea level.

Adaptation

It is estimated that 81.6 million people live at elevations above.
Genetic changes have been detected in high-altitude population groups in Tibet in Asia, the Andes of the Americas, and the Amhara of Ethiopia. This adaptation means irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes. The indigenous inhabitants of these regions thrive well in the highest parts of the world. These humans have undergone extensive physiological and genetic changes, particularly in the regulatory systems of oxygen respiration and blood circulation, when compared to the general lowland population.
Compared with acclimatized newcomers, native Amhara, Andean and Himalayan populations have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise. Tibetans demonstrate a sustained increase in cerebral blood flow, elevated resting ventilation, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness. Andeans possess a similar suite of adaptations but exhibit elevated hemoglobin concentration and a normal resting ventilation. These adaptations may reflect the longer history of high altitude habitation in these regions.
A lower mortality rate from cardiovascular disease is observed for residents at higher altitudes. Similarly, a dose–response relationship exists between increasing elevation and decreasing obesity prevalence in the United States. This is not explained by migration alone. On the other hand, people living at higher elevations also have a higher rate of suicide in the United States. The correlation between elevation and suicide risk was present even when the researchers control for known suicide risk factors, including age, gender, race, and income. Research has also indicated that oxygen levels are unlikely to be a factor, considering that there is no indication of increased mood disturbances at high altitude in those with sleep apnea or in heavy smokers at high altitude. The cause for the increased suicide risk is as yet unknown.