Man at High Altitudes
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Transcript of Man at High Altitudes
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Man at High Altitudes
• Atmosphere controls ability to live at high altitudes– Cold temperature– Low humidity– Low oxygen
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Physiological Responses to Cold Environments
• Homeostasis- Warm-blooded mammals maintain a relatively constant body temperature regardless of ambient conditions- humans 37oC
• Homeostasis achieved by control mechanisms that regulate heat production and loss
• Core body temperature drop of a few degrees reduces enzymatic activity, coma, death
• Core body temperature increases of a few degrees may irreversibly damage the central nervous system
• C Van Wie (1974) Physiological response to cold environments. Arctic & Alpine Enviornments
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Adaptation to Cold Environments
• To maintain temperature:– Increase insulation– Increase heat production– Lower core temperature (hypothermia)
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Thermoregulation
• Heat produced by metabolic processes and muscular exertion– Inactive
• Brain 16%
• Chest and abdomen 56%
• Skin and muscles 18%
– Active• Brain 3%
• Chest and abdomen 22%
• Skin and muscles 73%
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Thermoregulation
• Heat lost from body core to muscle and skin by conduction and convection
• Blood circulating through body carries heat from core to outer body– Some lost to air
– Much of the heat transferred to cooler veinous blood returning from extremities
– Enables body to maintain extremities at lower temperature
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Thermoregulation
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Skin layer heat losses
• As air flow increases, convective heat loss from skin increases- windchill
• Evaporation• Predominant heat loss from skin in cold
environments is radiation– Nude, with skin temp 31C, radiates 116 Watts to room
with walls of 21C
– At rest, total heat production is 84 Watts
– Better put some clothes on
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Wind Chill Science
• http://windchill.ec.gc.ca/workshop/index_e.html?• http://windchill.ec.gc
.ca/workshop/papers/html/session_2_paper_1_e.html
• Bluestein, Maurice, Jack Zecher, 1999: A New Approach to an Accurate Wind Chill Factor. Bulletin of the American Meteorological Society: Vol. 80, No. 9, pp. 1893–1900.
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Pathologic Effects of Excessive Heat Loss
• If skin temperature < freezing for extended period:– Chilblains- red, swollen itching lesions between joints of
fingers– Trench foot- similar to chilblains except on foot
• If skin freezes– Frostbite- local burning and stinging followed by numbness
• Exposure- condition when body is not able to maintain a normal temperature– Core temp < 30C lose consciousness– Core temp < 27C heart ceases
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Physiological Response to Cold Stress
• Autonomic control measures respond to cold by:– Increasing heat production – Increasing insulation layers– Permit moderate hypothermia (lower core body
temperature)
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Heat Generation
• At rest, muscles provide 18% of total heat• Voluntary exercise- heat production increased 10
times• Involuntary exercise- shivering
– heat production increased 4-5 times – but 90% of heat produced by shivering lost by
convection because of body movements
• Non-shivering thermogenesis– Metabolism/hormones of body adjust and increase heat
production
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Insulation
• Initial reaction to cold– Blood vessels in extremities contract rapidly– Increases insulation of body
• Long term- more fat
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Physiological Factors of Altitude: Oxygen Deficiency
• Proportion of Oxygen in atmosphere- 21%• Partial pressure of Oxygen decreases with height in proportion to other gases• Lungs saturated with water vapor; reduces available oxygen• Oxygen in lungs: (ambient pressure – saturation water vapor pressure at body temp
(37C) (63 mb)) * .21• Sea level (1013 – 63 ) * .21 = 200 mb; 5000 m (540 – 63 ) * .21 = 100 mb• Hypoxia- intolerance to oxygen deficiency
– Humans can tolerate half sea level value indefinitely– Symptoms significant above 3000 m (133 mb of Oxygen)
• Standard Atmosphere varies with latitude (4000 m roughly 630 mb equatorward of 30o; 593 mb (winter)-616 mb (summer) at 60o
• Cyclone could drop pressure 10-20 mb; equivalent to several hundred meters in elevation
• Grover (1974); Man living at high altitudes. Arctic and Alpine Environments.
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Inspired Oxygen as a Function of Elevation
200mb
100mb
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Supplemental Oxygen
• Mt. Everest (8848 m/29,028 ft)– Mean pressure near 314 mb– Most climbers use bottled oxygen above 7300
m (24,000 ft)
• Pilots required to use supplemental oxygen above 3810 m (12,500 ft) for flights lasting more than 30 minutes
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Oxygen in the body
• PIO2- inspired oxygen- oxygen available in the lungs
• O2 transported in body by respiratory pigment haemoglobin in red blood cells– Lungs oxygenate blood
– Heart pumps blood through body
– High pressure of O2 in capillaries causes diffusion into tissue
• Sea-level- 100 ml of blood contains 20 ml of O2
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Physiological Adaptions to Hypoxia
• Reduced PIO2 reduces pressure of O2 in blood: PaO2
• Brain triggers respiratory muscles to bring greater volume of air into lungs with each breath
• Hyperventilation- increase volume of air inspired per minute offsets decrease in air density
• # O2 molecules taken into lungs per minute is nearly same as at sea level
• However, while quantity of O2 available in lungs remains unchanged, PaO2 reduced as elevation increases
• Reduced PaO2 haemoglobin binds less O2; less saturation of O2 in blood; reduces O2 in blood
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Oxygen Saturation
70 116 mb
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Haemoconcentration
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Other physiological changes• Decrease in Oxygen in blood causes heart rate to increase initially in
order to maintain Oxygen transport
• Amount of water in blood plasma decreases after about a week
– Decreases plasma volume without changing volume of red blood cells
– Blood can carry greater quantity of Oxygen
– Prolonged hypoxia stimulates bone marrow to produce more red blood cells
• After a week, heart rate normalizes but stroke volume (volume pumped by left ventricle) decreases, leading to net drop in cardiac oxygen output
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VO2
• Highest pressure in O2 transport system determines efficiency of system
• VO2- aerobic working capacity- maximum amount of O2 that can be consumed per minute
• 10% decrease in VO2 per 1000m increase in altitude above 1500 m
• Humans can’t work as hard at high elevation as at lower ones
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VO2
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Problems at High Altitude
• Humans can adapt to altitudes of 3-4 km and remain healthy indefinitely
• Acute mountain sickness- initial response to rapid ascent to high elevation– Poor sleep; headaches; nausea; vomiting; apathetic; irritable; little
appetite
• Chronic mountain sickness- develops in people who have lived at high elevation for years; lose adaptation to hypoxia
• Pulmonary Oedema– Accumulation of fluids in the lungs interrupts transfer of oxygen
from air to blood
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Athletic Use of Hypoxia
http://www.sltrib.com/2001/aug/08262001/sports/126267.htm