Effects of high altitude on humans
Encyclopedia
The effects of high altitude
on humans are considerable. The percentage saturation of hemoglobin
with oxygen determines the content of oxygen in our blood. After the human body
reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to plummet. 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. Athletes use these adaptations to help their performance. There is a limit to the level of adaptation: mountaineers refer to the altitudes above 8000 metres (26,246.7 ft) as the "death zone", where no human body can acclimatize.
, where the atmospheric pressure
is about 100,000 Pa
or 1000 millibars
(or 1 atm
, by definition). The concentration of oxygen
(O2) in sea-level air is 20.9 %, so the partial pressure
of O2 (pO2) is about 21.2 kPa. In healthy individuals, this saturates hemoglobin
, the oxygen-binding red pigment in red blood cells.
Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about 100 km, so pO2 decreases exponentially with altitude as well. It is about half of its sea-level value at 5000 m (16,404.2 ft), the altitude of the Everest Base Camp
, and only a third at 8848 m (29,028.9 ft), the summit of Mount Everest
. When pO2 drops, the body responds with altitude acclimatization.
Mountain medicine recognizes three altitude regions that reflect the lowered amount of oxygen in the atmosphere:
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
(HAPE) and high altitude cerebral edema
(HACE). The higher the altitude, the greater the risk. Research also indicates elevated risk of permanent brain damage in people climbing to extreme altitudes.
Expedition doctors commonly stock a supply of dexamethazone, or "dex," to treat these conditions on site.
Humans have survived for two years at 5950 m (19,521 ft) [475 millibars of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude. At extreme altitudes, above 7500 m (24,606.3 ft) [383 millibars of atmospheric pressure], sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.
Finally, the death zone, in mountaineering
, refers to altitude
s above a certain point where the amount of oxygen
is not high enough to sustain human
life. This point is generally tagged as 8000 m (26,246.7 ft) [less than 356 millibars of atmospheric pressure]. The term "death zone" was originally coined by Edouard Wyss-Dunant
, a Swiss doctor, in his 1952 book, The Mountain World.
Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents).
In the "death zone", no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished. An extended stay in the zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness and, ultimately, death.
Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance to hypoxia in patients with Chronic mountain sickness
and normal fetuses in-utero, both of which present PaO2 levels similar to those at the summit of Mount Everest.
for permanent residents at higher altitudes. One exception to this trend is that people living at higher elevations have a statistically significant higher rate of suicide
. Similar findings were observed for both firearm
-related suicides (59% of suicides) and nonfirearm-related suicides. 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 also indicates 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 unknown so far.
). However, hyperventilation 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 (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves).
Full acclimatization, however, 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 is greatly enhanced by acetazolamide
. Eventually, the body has lower lactate
production (because reduced glucose breakdown decreases the amount of lactate formed), decreased plasma
volume, increased hematocrit
(polycythemia
), 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 kilometers by 11.4 days. For example, to adapt to 4000 metres (13,123.4 ft) of altitude would require around 46 days. The upper altitude limit of this linear relationship has not been fully established.
Athletes can also take advantage of altitude acclimatization to increase their performance. The same changes that help the body cope with high altitude increase performance back at sea level. However, this may not always be the case. Any positive acclimatization effects may be negated by a de-training effect as the athletes are usually not able to exercise with as much intensity at high altitudes compared to sea level.
This conundrum led to the development of the altitude training modality known as "Live-High, Train-Low", whereby the athlete spends many hours a day resting and sleeping at one (high) altitude, but performs a significant portion of their training, possibly all of it, at another (lower) altitude. A series of studies conducted in Utah in the late 1990s by researchers Ben Levine, Jim Stray-Gundersen, and others, showed significant performance gains in athletes who followed such a protocol for several weeks. Other studies have shown performance gains from merely performing some exercising sessions at altitude, yet living at sea level.
The performance-enhancing effect of altitude training could be due to increased red blood cell count, more efficient training, or changes in muscle physiology.
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
on humans are considerable. The percentage saturation of hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
with oxygen determines the content of oxygen in our blood. After the human body
Human body
The human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 100 trillion cells, the basic unit of life...
reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to plummet. 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. Athletes use these adaptations to help their performance. There is a limit to the level of adaptation: mountaineers refer to the altitudes above 8000 metres (26,246.7 ft) as the "death zone", where no human body can acclimatize.
Effects as a function of altitude
The human body functions best at sea levelSea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
, where the atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
is about 100,000 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
or 1000 millibars
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
(or 1 atm
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
, by definition). The concentration of oxygen
Atmospheric chemistry
Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary field of research and draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and...
(O2) in sea-level air is 20.9 %, so the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of O2 (pO2) is about 21.2 kPa. In healthy individuals, this saturates hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, the oxygen-binding red pigment in red blood cells.
Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about 100 km, so pO2 decreases exponentially with altitude as well. It is about half of its sea-level value at 5000 m (16,404.2 ft), the altitude of the Everest Base Camp
Everest Base Camp
There are two base camps on opposite sides of Mount Everest. South Base Camp is in Nepal at an altitude of , and North Base Camp is in Tibet at ). These camps are rudimentary campsites on Mount Everest that are used by mountain climbers during their ascent and descent...
, and only a third at 8848 m (29,028.9 ft), the summit of Mount Everest
Mount Everest
Mount Everest is the world's highest mountain, with a peak at above sea level. It is located in the Mahalangur section of the Himalayas. The international boundary runs across the precise summit point...
. When pO2 drops, the body responds with altitude acclimatization.
Mountain medicine recognizes three altitude regions that reflect the lowered amount of oxygen in the atmosphere:
- High altitude = 1500–3500 m (4,921.3–11,482.9 ft)
- Very high altitude = 3500–5500 m (11,482.9–18,044.6 ft)
- Extreme altitude = above 5500 metres (18,044.6 ft)
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
High Altitude Pulmonary Edema
High altitude pulmonary edema is a life-threatening form of non-cardiogenic pulmonary edema that occurs in otherwise healthy mountaineers at altitudes typically above ....
(HAPE) and high altitude cerebral edema
High altitude cerebral edema
High altitude cerebral edema is a severe form of altitude sickness. HACE is the result of swelling of brain tissue from fluid leakage and almost always begins as acute mountain sickness...
(HACE). The higher the altitude, the greater the risk. Research also indicates elevated risk of permanent brain damage in people climbing to extreme altitudes.
Expedition doctors commonly stock a supply of dexamethazone, or "dex," to treat these conditions on site.
Humans have survived for two years at 5950 m (19,521 ft) [475 millibars of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude. At extreme altitudes, above 7500 m (24,606.3 ft) [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
Mountaineering
Mountaineering or mountain climbing is the sport, hobby or profession of hiking, skiing, and climbing mountains. While mountaineering began as attempts to reach the highest point of unclimbed mountains it has branched into specialisations that address different aspects of the mountain and consists...
, refers to altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
s above a certain point where the amount of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is not high enough to sustain human
Human
Humans are the only living species in the Homo genus...
life. This point is generally tagged as 8000 m (26,246.7 ft) [less than 356 millibars of atmospheric pressure]. The term "death zone" was originally coined by Edouard Wyss-Dunant
Edouard Wyss-Dunant
Dr. Edouard Wyss-Dunant is a Swiss physician and alpinist. He had a distinguished career in medicine, both in his own country and abroad. He published a number of treatises in hi s professional capacity and was the author of several mountaineering books...
, a Swiss doctor, in his 1952 book, The Mountain World.
Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents).
In the "death zone", no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished. An extended stay in the zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness and, ultimately, death.
Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance to hypoxia in patients with Chronic mountain sickness
Chronic mountain sickness
Chronic mountain sickness is a disease that can develop during extended time living at altitude. It is also known as 'Monge’s disease' , after its first description in 1925 by Carlos Monge. While acute mountain sickness is experienced shortly after ascent to high altitude, chronic mountain...
and normal fetuses in-utero, both of which present PaO2 levels similar to those at the summit of Mount Everest.
Long-term effects
There is a a significantly lower overall mortality rateMortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
for permanent residents at higher altitudes. One exception to this trend is that people living at higher elevations have a statistically significant higher rate of suicide
Suicide
Suicide is the act of intentionally causing one's own death. Suicide is often committed out of despair or attributed to some underlying mental disorder, such as depression, bipolar disorder, schizophrenia, alcoholism, or drug abuse...
. Similar findings were observed for both firearm
Firearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...
-related suicides (59% of suicides) and nonfirearm-related suicides. 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 also indicates 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
Sleep apnea
Sleep apnea is a sleep disorder characterized by abnormal pauses in breathing or instances of abnormally low breathing, during sleep. Each pause in breathing, called an apnea, can last from a few seconds to minutes, and may occur 5 to 30 times or more an hour. Similarly, each abnormally low...
or in heavy smokers at high altitude. The cause for the increased suicide risk is unknown so far.
Acclimatization to altitude
The human body can adapt to high altitude through 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 rate (hyperventilationHyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
). However, hyperventilation also causes the adverse effect of respiratory alkalosis
Respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH...
, inhibiting the respiratory center
Respiratory center
The respiratory center is located in the medulla oblongata, which is the lowermost part of the brain stem. The RC receives controlling signals of neural, chemical and hormonal nature and controls the rate and depth of respiratory movements of the diaphragm and other respiratory muscles...
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
Stroke volume
In cardiovascular physiology, stroke volume is the volume of blood pumped from one ventricle of the heart with each beat. SV is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood in the ventricle at the end of a beat from the volume...
is slightly decreased; and non-essential bodily functions are suppressed, resulting in a decline in food digestion efficiency (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves).
Full acclimatization, however, 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 is greatly enhanced by acetazolamide
Acetazolamide
Acetazolamide, sold under the trade name Diamox, is a carbonic anhydrase inhibitor that is used to treat glaucoma, epileptic seizures, Idiopathic intracranial hypertension , altitude sickness, cystinuria, and dural ectasia...
. Eventually, the body has lower lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
production (because reduced glucose breakdown decreases the amount of lactate formed), decreased plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
volume, increased hematocrit
Hematocrit
The hematocrit or packed cell volume or erythrocyte volume fraction is the percentage of the concentration of red blood cells in blood. It is normally about 45% for men and 40% for women...
(polycythemia
Polycythemia
Polycythemia is a disease state in which the proportion of blood volume that is occupied by red blood cells increases...
), increased RBC
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
mass, a higher concentration of capillaries in skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
tissue, increased myoglobin
Myoglobin
Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
, increased mitochondria, increased aerobic enzyme concentration, increase in 2,3-BPG
2,3-Bisphosphoglycerate
2,3-Bisphosphoglyceric acid is a three-carbon isomer of the glycolytic intermediate 1,3-bisphosphoglyceric acid . 2,3-BPG is present in human red blood cells at approximately 5 mmol/L...
, hypoxic pulmonary vasoconstriction
Hypoxic Pulmonary Vasoconstriction
Hypoxic pulmonary vasoconstriction is a paradoxical, physiological phenomenon in which pulmonary arteries constrict in the presence of hypoxia without hypercapnia , redirecting blood flow to alveoli with a higher oxygen content.The process might at first seem illogical, as low oxygen levels should...
, and right ventricular hypertrophy
Right ventricular hypertrophy
Right ventricular hypertrophy is a form of ventricular hypertrophy affecting the right ventricle.Blood travels through the right ventricle to the lungs via the pulmonary arteries. If conditions occur which decrease pulmonary circulation, meaning blood does not flow well from the heart to the...
. 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 kilometers by 11.4 days. For example, to adapt to 4000 metres (13,123.4 ft) of altitude would require around 46 days. The upper altitude limit of this linear relationship has not been fully established.
Altitude and athletic performance
For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure means there is less resistance from the atmosphere and the athlete's performance will generally be better at high altitude. For endurance events (races of 5,000 metres or more), the predominant effect is the reduction in oxygen, which generally reduces the athlete's performance at high altitude. Sports organizations acknowledge the effects of altitude on performance: the International Association of Athletics Federations (IAAF), for example, have ruled that performances achieved at an altitude greater than 1,000 metres will not be approved for record purposes.Athletes can also take advantage of altitude acclimatization to increase their performance. The same changes that help the body cope with high altitude increase performance back at sea level. However, this may not always be the case. Any positive acclimatization effects may be negated by a de-training effect as the athletes are usually not able to exercise with as much intensity at high altitudes compared to sea level.
This conundrum led to the development of the altitude training modality known as "Live-High, Train-Low", whereby the athlete spends many hours a day resting and sleeping at one (high) altitude, but performs a significant portion of their training, possibly all of it, at another (lower) altitude. A series of studies conducted in Utah in the late 1990s by researchers Ben Levine, Jim Stray-Gundersen, and others, showed significant performance gains in athletes who followed such a protocol for several weeks. Other studies have shown performance gains from merely performing some exercising sessions at altitude, yet living at sea level.
The performance-enhancing effect of altitude training could be due to increased red blood cell count, more efficient training, or changes in muscle physiology.
See also
- Altitude sicknessAltitude sicknessAltitude sickness—also known as acute mountain sickness , altitude illness, hypobaropathy, or soroche—is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude...
- Altitude tentAltitude tentAn altitude tent, also known as an altitude simulation tent or a hypoxic tent, is an enclosed living space which simulates high altitude by maintaining a lower oxygen concentration. It is used by athletes and by high-altitude mountain climbers to stimulate the body's natural adaptations to...
- Armstrong's limit the altitude/pressure at which water boils in the lungs at body temperature
- Gamow bagGamow bagA Gamow bag is an inflatable pressure bag large enough to accommodate a person inside. By inflating the bag with a foot pump, the effective altitude can be decreased by 1000 to as much as 3000 meters . It is primarily used for treating severe cases of altitude sickness.The gamow bag was named after...
- 1996 Mount Everest disaster
- 2008 K2 disaster2008 K2 disasterThe 2008 K2 disaster occurred on August 1, 2008, when eleven mountaineers from international expeditions died on K2, the second-highest mountain on Earth. Three others were seriously injured. It was the worst single accident in the history of K2 mountaineering.-Expedition goal: K2:K2 is the...
- HypoxemiaHypoxemiaHypoxemia is generally defined as decreased partial pressure of oxygen in blood, sometimes specifically as less than or causing hemoglobin oxygen saturation of less than 90%.-Distinction from anemia and hypoxia:...
- Hypoxia (medical)Hypoxia (medical)Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
- Organisms at high altitudeOrganisms at high altitudeOrganisms can exist at high altitude in a habitat, while flying or gliding, or through man-made systems. Many animals have adapted to high altitude life and some have evolved to cope well with the problems of an environment with a reduced level of oxygen....
- Aviation medicineAviation medicineAviation medicine, also called flight medicine or aerospace medicine, is a preventive or occupational medicine in which the patients/subjects are pilots, aircrews, or persons involved in spaceflight...
External links
- IPPA, High Altitude Pathology Institute.