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"Brr! It's so cold outside" -

Human body responses to cold and hypothermia

Dr Sampurna Roy MD



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A cold and cloudy Sunday is an ideal day to write a post on the dangers of hypothermia and the response of our body to  cold temperature in general.      Doctor are you using social media ?

In most parts of the world the environmental temperature is well below that of deep body structures.

There must be active heat production to maintain the normal deep body temperature at a constant level.

If the environmental temperature falls greatly the body can help to keep its internal temperature constant by increasing heat production or by reducing heat loss.

1)  Heat production is increased by raising the metabolic rate.

Cold promotes hunger which increases food intake.

Secretions of adrenalin and of thyroid hormones produce small increase in metabolic rate.

Shivering and voluntary muscular activity release large amounts of heat in the muscles. 

In the newborn infant non-shivering thermogenesis in brown fat is promoted by sympathetic activity.

2) Heat loss is reduced by cutaneous vasoconstriction.

This reduces the amount of heat transferred by the blood from the body core to the body surface.

In the limbs a counter-current heat exchange between the warm arterial blood and the cooled blood in the venae comitantes further helps to conserve body heat.

The cutaneous vasoconstriction may raise the blood pressure.

3) Cutaneous vasodilatation.

Although most cutaneous vessels constrict in response to cold, the vessels of the hand, foot, ear, and face may exhibit 'cold vasodilatation'.

This occurs on exposure of these parts to moderate cold  (Example: 5-10 degree C) when the body is generally warm. 

It is regarded as a good response which will prevent the development of frostbite.

The cold vasodilatation response increases as a person becomes acclimatized to cold.

4) A layer of subcutaneous fat.

Heat loss is also reduced by a thick layer of subcutaneous fat which provides excellent insulation. This may explain the endurance by long-distance swimmers of temperature of 15 degree C. for several hours.

An extra 1mm of subcutaneous fat is equivalent to raising the temperature of the water by 1.5 degree C.


Adaptation to cold

Without protection by clothing, shelter and heating man cannot survive in the polar regions of Earth.

No physiological adaptation could enable a naked man to withstand temperature of -100 degree C or severe blizzards. If artificial insulation is really effective physiological adaptation does not occur.

The Eskimos' igloo is made of hard snow which is an excellent insulator as it is made up of countless number of small air cells. 

The oil lamp used for heating melts the inner layer of snow which refreezes to form a smooth reflecting surface which conserves radiant heat.

The temperature rises to about 30 degree C.



The modern polar hut has an outer weatherproof skin, walls made of material layered to trap pockets of air and an internal lining of aluminium foil to reflect radiant heat from diesel stoves.

Men at polar regions are out of doors for less than 10 per cent of the time and general adaptation to cold takes place.

Eskimos and Lapps have a higher blood flow through the hands than other men living under the same conditions.

This may be related to the raised metabolic rate brought about by a high protein diet.

True adaptation to cold occurs among the Australian aborigines and Kalahari bushmen. They can sleep almost naked, without shivering, at air temperature down to 4 degree C and with a skin temperature of  27 - 28 degree C.

In these conditions there is some fall in body temperature but this is limited by marked cutaneous vasoconstriction.

This process is called "insulative hypothermic acclimatization" and is well suited for those parts of the world in which people are undernourished and exposed to high temperatures during the day time.

By contrast a naked man in the developed world shivers when the air temperature drops below 27 degree C  or the skin temperature is below 33 degree C.

The onset of shivering prevents a fall in body temperature but the discomfort prevents sleep.


Types of Hypothermia:

I Natural:

Hypothermia occurs naturally in hibernating animals in which with the onset of hibernation, the body temperature varies with that of the environment over the range 5 to 15 degree C.

II Accidental:


 Visit: Falling Sick in the Mountains - Pathology of Altitude Related Illness.

In human accidental hypothermia occurs in the following conditions:

1)  Accidental hypothermia may occur in healthy people who are exposed to excessive cooling.

Immersion in the cold sea after shipwreck or exposure of inadequately clothed person on mountains can overcome the capacity of the thermoregulatory mechanisms so that the deep body temperature falls.

At 33 degree C. temperature regulation fails, and at 29-31 degree C. consciousness is lost.

Below 30 degee C. glucose is metabolized slowly or not at all, breathing is depressed and the heart rate is very slow.

Death occurs at body temperatures below 25 degree C.

2)  Accidental hypothermia also occurs in elderly persons living in inadequately heated rooms.

Old people when exposed to the cold show a smaller increase in O2 consumption, a greater fall of rectal temperature and less cutaneous vasoconstriction than young people.

Many geriatric patients die from hypothermia every winter.

3)  The development of hypothermia may be accelerated by drugs such as chlorpromazine and sedatives.

4)  A few patients suffer from chronic hypothermia in which the defects is a low setting of the hypothalamic thermostat.

Temperature regulation occurs but the usual level is abnormally low.

In these cases there is no evidence of endocrine disease.

Hypothermia occurs in myxoedema because the patients cannot increase their metabolic rate on exposure to cold.

III Deliberate :  

Hypothermia is deliberately induced in some patients undergoing operations on the heart and brain.

The body temperature is lowered to 21-25 degree C. by cooling the blood or the surface of the body, and at these temperatures the O2 usage by the tissues falls to about a quarter of the normal. 

Cardiac arrest for 10 minutes under these circumstances does not cause permanent damage to the heart or brain and allows the performance of operations on these organs which would be impossible without cooling.

Certain drugs have been used to facilitate the lowering of body temperature.

For example, chlorpromazine (Largactil) increases heat loss through cutaneous vasodilation, brought about by central inhibition of vasomotor tone and by peripheral antagonism of noradrenalin and adrenalin.


Clothing in cold climates

- The purpose of clothing as a protection against cold is to provide a layer of thermal insulation to reduce the amount of heat lost from the body surface to the environment.



The insulation is provided by the 'dead' air trapped in the clothing.

Animal furs are excellent in this respect for their original owners and unfortunately also for human beings.

This was long recognized by Eskimos who wear two layers of fur, one facing outwards and the other facing inwards.

- The maximum thickness of dead air should be maintained even on movement.

For example, gloves should be made with curved fingers since the fingers are more commonly bent than straight.

When the fingers are bent inside ordinary straight fingered gloves the dead air layer at the flexures is much narrowed and thermal insulation is reduced.

- While exercising during cold climate the movements of air inside the clothing should increase heat loss from the body surface.

If the heat loss is inadequate sweating occurs and is very uncomfortable in these circumstances.

- The ideal clothing for cold climates should include materials which are permeable to water and impermeable to air movements.

Such features are possible because movement of air through a material is a different process from diffusion of water vapour.

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Visit: Falling Sick in the Mountains - Pathology of Altitude Related Illness.

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Dr  Sampurna Roy  MD

Consultant Histopathologist (Kolkata - India)







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