300cal./m^/hr. These high rates cannot be maintained for long periods and the ave- 

 average increment over a period of one hour is unlikely to exceed 150- 200 cal./m^/hr. 



If the cold environment is maintained, shivering gradually diminishes as exhaus- 

 tion proceeds and the deep body temperature will begin to drop. When body cooling 

 is accelerated by immersion in cold water, it is found that shivering ceases at a 

 rectal temperature of approximately 32- 33°C, and metabolic rate declines thereafter 

 with rectal temperature. Conciousness is lost at a rectal temperature of 30°C and 

 death usually occurs at rectal temperature of 25°C, although survival has been re- 

 ported in one subject whose rectal temperature was below 20°C. 



There are many other physiological changes which occur during acute exposure 

 to cold which can only be briefly summarised. The effect is to diminish heat loss 

 by increasing insulation, which is mainly effected by vasoconstriction in the skin 

 and underlying muscles. The reduction in blood flow leads to a fall in skin tempera- 

 ture, and the gradient of temperature from the deep tissues to the surface becomes 

 steeper. Body hair is vestigial inman^but the pilo-arrectores muscles attached to 

 the roots of the hairs contract and so produce goose flesh in the skin. This roughen- 

 ing of the skin surface increases the boundary layer of air in contact with the body 

 and so has a small effect on insulation. Water loss from the skin surface is greatly 

 reduced. 



Acute exposure to cold also stimulates certain endocrine changes, similar to 

 those described by the term 'alarm reaction*. The main characteristic of this reac- 

 tion is the increased activity of the adreno- cortical mechanism. 



The effects of long continued exposure to cold environments include vascular 

 and endocrine changes. As a result of peripheral vasoconstriction, there is a shift 

 of blood from the superficial regions of the body to the pulmonary and probably the 

 splanchnic areas. In addition, there is a gradual diminution of the blood volume, 

 owing to the loss of plasma with consequent haemoconcentration. The proportion of 

 red cells to plasma increases from a normal of 46% up to 52- 55%. During the period 

 of haemoconcentration there is a marked increase of urine secretion, and this diure- 

 sis represents a period of increased water loss. 



The endocrine changes in man cannot be adequately described at present. In 

 laboratory animals who are exposed for long periods to temperatures of 0°C, there is 

 an increased activity of the thyroid gland, which is accompanied by a gradual rise 

 in the basal metabolic rate. 



In laboratory animals there is also a hypertrophy of the cortex of the adrenal 

 gland, which can be diminished by increasing the amounts of ascorbic acid in the 

 diet. 



The main physiological problem as far as man is concerned is whether acclima- 

 tization to cold takes place, in the sense that physiological changes occur which in- 

 crease tolerance for cold or improve survival in the cold. Such changes can be 

 clearly demonstrated in laboratory animals. Blair, for example, has recently com- 

 pared the responses of control rabbits with animals kept in the cold chamber for 

 many weeks. When both were exposed to very severe cold, the control animals all 



208 



