Man at High Altitudes

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/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.

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