Hypoxic ventilatory response
Hypoxic ventilatory response (HVR) is the increase in
In
The physiological mechanisms differ in effect and in course of time. HVR is time dependent and can be divided into two phases: the first (0–5 minutes) of ventilation increase, and the second (5–20 minutes) of slow decline.[4]
The initial increase in ventilation from HVR is initiated by the
As HVR is a response to decreased oxygen availability,[1] it shares the same environmental triggers as hypoxia. Such precursors include travelling to high altitude locations[6] and living in an environment with high levels of carbon monoxide.[7] Combined with climate, HVR can affect fitness and hydration.[2] Especially for lowlanders who traverse past 6000 meters in altitude, the limit of prolonged human exposure to hypoxia, HVR may result in hyperventilation and ultimately the deterioration of the body. Oxygen consumption is reduced to a maximum of 1 liter per minute.[8]
Travelers acclimatized to high altitudes exhibit high levels of HVR, as it provides advantages such as increased oxygen intake, enhanced physical and mental performance, and lower susceptibility to illnesses associated with high altitude.
Physiology
Acute hypoxic ventilatory response
Acute response (AR)
The first stage of the hypoxic ventilatory response consists of the initial reaction to a hypoxic environment leading up to the peak known as short-term potentiation (STP).[11] The process is induced by a decrease in oxygen partial pressure in blood. Type I glomus cells of carotid bodies detect the change in oxygen levels and release neurotransmitters towards the carotid sinus nerve, which in turn stimulates the brain, ultimately resulting in increased ventilation.[2] The period of increased ventilation varies among different individuals but typically lasts under ten minutes.[12]
Short-term potentiation (STP)
STP is the increase in ventilation after the acute hypoxic response and the eventual return of ventilation to its equilibrium after
Short-term depression (STD)
STD is a temporary jump in respiratory frequency at the beginning of carotid chemo afferent stimulation or a temporary drop in respiratory frequency at the end of chemo afferent stimulation. This mechanism lasts from a span of several seconds to a few minutes.[14] STP has only been found in the respiratory frequency of phrenic nerve stimulation, which produces contraction of the diaphragm.
Ventilatory response to sustained hypoxia
A continued presence in a
Chronic hypoxic ventilatory response
Chronic hypoxia results in further physiological changes due to the transcription factor
Neurology
The
High altitude adaptation
Populations residing in altitudes above 2,500 meters have adapted to their hypoxic environments.
Anthropology
Populations
Andeans
The Andean peoples are one of three central populations of study that have a decreased HVR. These populations notably inhabit areas in and around the Andes mountain range, which has an average altitude of 13,000 feet (4,000 m).[21] HVR has been studied in inhabitants of Cusco, Peru, which lies at 11,000 feet (3,400 m).[21] Living in such high altitudes has led to cultural adaptations, including the consumption of coca tea. Coca tea is an extract made by boiling the leaves of the coca plant in water and contains the stimulant Cocaine. For millennia, Andeans have used coca tea as a treatment for acute altitude sickness,[22] and to this day it is still given to those travelling to the high altitude regions of Peru, though, its effectiveness has been disputed.[23] In a 2010 study published in the Journal of Travel Medicine, the consumption of coca tea was actually associated with an increase in the incidence of altitude sickness experienced by travelers visiting the city of Cusco, Peru.[23]
It has been found that the ventilatory response is substantially less pronounced in the Andean populations than in the Tibetans, with the HVR response of Tibetans roughly double that of Andeans at an altitude of around 4,000 metres (13,000 ft).[24] The altitude adaptations also appear to be less permanent than those seen in the Tibetan populations, as the Andeans have a much higher prevalence of Chronic Mountain Sickness (CMS), where the body develops a harmful reaction to low oxygen levels over many years.[25]
Tibetans
The Tibetan people are an ethnic group native to Tibet that live throughout the Tibetan Plateau. They live at altitudes up to 15,000 feet (4,600 m),[26] and are thus of extreme interest to researchers investigating HVR in high altitude populations. One of these populations are the Sherpa people, a group of Tibetans who are sought after for their knowledge of and skill with navigating through the Himalayas. Historically, Sherpas have been contracted to guide expeditions up Mount Everest, but the practice has since declined in light of exploitation of the Sherpa guides. The energy and ease at which the Sherpa ascend and descend mountains is due to their ability to use oxygen more efficiently.[27] This ability to excel at mountaineering has shifted their culture around it. Tourism has become a driving force for the financial income of the Sherpa people. The Sherpa are able to make much more money[28] acting as travel guides due to their local knowledge, and climbing ability.
Genetic evidence suggests that the Tibetan peoples diverged from the larger Han Chinese population any time around 1,000 B.C.E.[29][30][31] to 7,000 B.C.E.[32][33] Given the significant mutations to the EPAS1 gene that contribute to the Tibetan resistance to altitude sickness, this suggests that the extreme evolutionary pressure on the Tibetan peoples has produced one of the fastest natural selection effects seen in a human population.[34] The adaptations of Tibetans to their hypoxic ventilatory response interact with other adaptations to promote successful reproduction. For example, Tibetans have evolved a greater oxygen saturation during infancy, leading to a lower rate of child mortality than experienced by non-adapted populations at altitude.[35]
Amhara
The
References
- ^ a b c Cymerman, A; Rock, PB. "Medical Problems in High Mountain Environments. A Handbook for Medical Officers"[usurped]. USARIEM-TN94-2. US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report. Retrieved 2009-03-05.
- ^ a b c d e f g Teppema, Luc J., and Albert Dahan. "The ventilatory response to hypoxia in mammals: mechanisms, measurement, and analysis." Physiological Reviews 90.2 (2010): 675-754.
- ^ Stanford, Craig, John S. Allen, and Susan C. Anton. Biological Anthropology : The Natural History of Humankind. 2nd ed. Upper Saddle River: Prentice Hall Higher Education, 2008. 151-52.
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- ^ a b Hornbein, Thomas F., and Robert B. Schoene. High Altitude: An Exploration Of Human Adaptation. n.p., New York: Marcel Dekker, c2001., 2001. OskiCat. Web. 8 Nov. 2016.
- ^ "Altitude Hypoxia Explained." Archived 10 November 2016 at the Wayback Machine Altitude Research Center. Altitude Research Center, n.d. Web. 08 Nov. 2016.
- ^ Karius, Diane R. "Respiratory Adaptations in Health and Disease: Forms of Hypoxia." Forms of Hypoxia. Kansas City University, n.d. Web. 08 Nov. 2016.
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- ^ Beall Cynthia M. "Tibetan and Andean patterns of adaptation to high-altitude hypoxia". Human Biology. 2000: 201–228.
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- ^ a b "Andes Mountains | mountain system, South America". Encyclopædia Britannica. Retrieved 10 November 2016.
- ^ Rottman, April (9 December 1997). "Erythroxylum: The Coca Plant". Retrieved 11 November 2016.
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- ^ "Plateau of Tibet | plateau, China". Encyclopædia Britannica. Retrieved 10 November 2016.
- ^ Meera Senthilingam, for (11 November 2015). "Scientists discover why Sherpas are superhuman climbers - CNN.com". CNN. Retrieved 11 November 2016.
- ^ "Guide: What does a Sherpa at Mount Everest do? - CBBC Newsround". 23 April 2014. Retrieved 11 November 2016.
- ^ Sanders R (1 July 2010). "Tibetans adapted to high altitude in less than 3,000 years". News Centre, UC Berkeley. UC Regents. Retrieved 8 July 2013.
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