Man at High Altitudes
• Atmosphere controls ability to live at high
altitudes
– Cold temperature
– Low humidity
– Low oxygen
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
Adaptation to Cold Environments
• To maintain temperature:
– Increase insulation
– Increase heat production
– Lower core temperature (hypothermia)
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%
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
Thermoregulation
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
Wind Chill Science
• http://windchill.ec.gc.ca/workshop/index_e.html?
• http://windchill.ec.gc.ca/workshop/papers/html/ses
sion_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.
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
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)
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
Insulation
• Initial reaction to cold
– Blood vessels in extremities contract rapidly
– Increases insulation of body
• Long term- more fat
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.
Inspired Oxygen as a Function of Elevation
200mb
100mb
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
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
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
Oxygen Saturation
70
116 mb
Haemoconcentration
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
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
VO2
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
Athletic Use of Hypoxia
http://www.sltrib.com/2001/aug/08262001/sports/126267.htm