What is high altitude?
Altitude is defined as the height of any object or reference above sea level (1). Altitude is typically talked about in regards to certain places or locations, such as the altitude of New Orleans, Louisiana or Mount Everest in Tibet China (1). As the pilot of an airplane turns the seatbelt sign off, altitude is also commonly talked about in reference to a “cruising altitude,” which is simply just the altitude that jets and engines ideally fly at. To put ‘altitude’ into a little bit of perspective, in Keene, New Hampshire, we are currently sitting at an altitude of 509 feet above sea level, while New Orleans is at a measly altitude of 3 feet above sea level, and Mount Everest staggers at 29,035 feet above sea level (2) (3). The average “cruising altitude” set by air traffic control towers for most planes is around 39,000 feet, but can vary on length of flight (4).
It is important to understand that at higher altitudes, the percent of oxygen present is equal to other varying altitudes, but the amount of oxygen decreases at higher altitudes (5). This is because there is a decrease in the atmospheric pressure, which decreases the partial pressure of oxygen in the lungs, ultimately decreasing the amount of gas exchange in the lungs (5). According to Boyle’s Law, PV= k which states the inverse relationship between pressure and volume; as pressure increases, volume decreases, and vice versa (5). Therefore, at higher altitudes, the atmospheric pressure decreases, and volume increases, and when volume increases, the air becomes thinner (5). With the same amount of oxygen present in a larger volume, the oxygen molecules are more dispersed, ultimately decreasing the concentration of the oxygen molecules present at higher altitudes (5). When humans make the courageous decision to hike mountains such as Mount Everest, or move to Colorado, they experience the effects of the lack of oxygen, and this can sometimes be called altitude sickness, and in order to overcome these symptoms, the human body must acclimate (5). Human populations that have lived in higher altitude conditions for several years have been able to genetically adapt.
What is Acute Hypoxia?
Acute hypoxia is defined as the sudden depletion or deficiency of available oxygen, and acute hypoxemia is specifically low oxygen levels in blood (9). The lack of oxygen in the body can cause a series of problems, including the initial shortness of breath (9). Other symptoms of acute hypoxia include changes of color in your skin to a red or blue, confusion, increase in heart rate, wheezing, coughing, lethargy, inability to talk, irritability, anxiety (9)(10). If not treated, acute hypoxia or hypoxemia can lead to coma or death (10).
Acclimation to High Altitudes
As previously mentioned, at higher altitudes there is decreased amounts of oxygen, and thus making it harder for your body to get the oxygen it needs. Therefore, the human body has this magnificent feature that allows us to acclimate to different environmental changes, such as high altitude, however time is the consequence (6). For example, to stick with the theme, you are hiking Mount Everest, and you are at an altitude of 15,000 ft, it can take 1-3 days to for acclimatization to occur (6). However, this means that when you move to a higher altitude, say at 18,000 feet, you will then have to pause again for another 1-3 days, so your body can acclimate again to the decrease in oxygen it’s getting (6).
But how does this happen, how does your body acclimate? There are several mechanisms that have evolved, that allow humans to quickly acclimate. The simplest, and easiest way to get more oxygen on board, is to increase the number of breaths you take, and the depth of the breaths you take (6). This makes sense, when you are on a run, your body needs more oxygen on board, so you can continue running, so what do you do? You breathe heavier and faster to make up for that deficit. The next change that occurs, is your body starts producing more red blood cells, so that it can carry more oxygen, and bring more of that oxygen to necessary tissues (6). Another change that goes hand-in-hand with the last change, is that hemoglobin, a protein inside a red blood cell, allows for a more cooperative, or quicker release of oxygen from hemoglobin, so it can disperse to the body’s tissues (6). One last mechanism that I will mention, in which our bodies acclimate to the lower oxygen levels, is that the pressure in our pulmonary arteries also increase (6). An increase in pulmonary pressure forces blood into different portions of the lungs that typically are not utilized at normal altitudes, ultimately increasing gas exchange and oxygen delivery (6).
Adaptation to High Altitudes
Human populations that have lived at higher altitudes for several hundreds of years have genetically evolved to the chronic hypoxia one would experience if they moved to one of these locations. Of these locations, Tibet, where Mount Everest is located, has a human population that has genetically evolved through the hypoxia-inducible factor (HIF) pathway (7). The hypoxia-inducible factor (HIF) pathway has been linked to the central pathway that regulates any changes in oxygen tension, which ultimately, contributes to oxygen delivery (7). In these populations at higher altitudes, not just the Tibetan population, but also the populations of Andean Altiplano and Semien Plateau of Ethiopia, the HIF pathway has undergone some genetic changes that leave these populations at a huge advantageous adaptation in regards to chronic hypoxia (7). Two specific genes in the Tibetan population called PHD2 and HIF2A, have been found to respond to the issue of chronic hypoxia (7). These populations therefore do not experience chronic hypoxia like their ancestors once did. This is unfortunately just one example explained here among several, of a genetic mutation that has been selected over the past hundreds of years in response to a single effect caused by high altitude to a population-there are many, many more.
What’s the Big Deal?
It is important as an avid hiker or traveler to understand the effects of high altitude, and to prepare for the potential effects of acute hypoxia, and prepare your trips with enough time and supplies accordingly. Unfortunately, some of these effects can cause some pretty unexpecting symptoms, and something as simple as higher altitudes than what we are used to can be the difference between life and death. Our bodies are made to acclimate and populations are made to adapt, but time will always be a factor that must be considered.
References
(1) Altitude. (n.d.). Retrieved November 29, 2017, from http://www.dictionary.com/browse/altitude
(2) Data, U. C. (n.d.). Temperature – Precipitation – Sunshine – Snowfall. Retrieved November 28, 2017, from https://www.usclimatedata.com/map.php?location=USNH0119
(3) Mount Everest Information . (n.d.). Retrieved November 29, 2017, from http://www.teameverest03.org/everest_info/
(4) Wachman, M. (n.d.). What Is the Altitude of a Plane in Flight? Retrieved November 29, 2017, from http://traveltips.usatoday.com/altitude-plane-flight-100359.html
(5) Peacock, A. J. (1998). Oxygen at high altitude. BMJ : British Medical Journal, 317(7165), 1063–1066.
(6) Curtis , R. (1995). OA Guide to High Altitude: Acclimatization and Illnesses. Retrieved November 29, 2017, from https://www.princeton.edu/~oa/safety/altitude.html
(7) Bigham, A. W., & Lee, F. S. (2014). Human high-altitude adaptation: forward genetics meets the HIF pathway. Genes & Development, 28(20), 2189–2204. http://doi.org/10.1101/gad.250167.114
(8) Hypoxia. (n.d.). Retrieved December 11, 2017, from https://www.merriam-webster.com/dictionary/hypoxia
(9) Hypoxia and Hypoxemia. (n.d.). Retrieved December 13, 2017, from https://www.webmd.com/asthma/guide/hypoxia-hypoxemia#1
(10) Hypoxia and Hypoxemia. (n.d.). Retrieved December 13, 2017, from https://www.medicinenet.com/hypoxia_and_hypoxemia/article.htm#what_are_the_symptoms_of_hypoxia_and_hypoxemia
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