This simplified account indicates that the maximum ability of the human body on 

 physical grounds to maintain heat balance demands an output of about 1 litre/hr sweat 

 for a cooling equivalent to 500 Kcals/hr. (It is as if the provision of sweat glands 

 had been evolved to cope with heat loads as high as that required for life and acti- 

 vity in hot dry regions rather than for the lower heat loads of hot humid tropics). In 

 actual tests on Europeans this sweat rate (1 litre/hr) is indeed about the limit of 

 what the sweat glands can manage to maintain for 4—6 hours. It is not surprising 

 that physiological acclimatization, as we shall see, improves the performance of the 

 sweat glands and that efficient performance as well as breakdown in the desert for 

 the most part is a matter of water supply and water metabolism as abundantly illus- 

 trated by the work of Adolph and his colleagues. 



4. Some Physiological Adjustments 



The changes which we know occur in the heat regulatory system, the circulation, 

 the kidney and the endocrine system cannot be dealt with in any detail here. But it 

 is important to realise that short term exposure to heat induces circulatory effects 

 primarily to facilitate a greatly increased loss of heat from the surface, and as this 

 depends so much on sweating, there are consequent adjustments in water (and salt 

 balance) throughout the body. It is the regulation of these that calls for endocrine 

 activity by the posterior pituitary and the adrenal cortex — to mention only those 

 glands for which we have evidence. 



Salt Intake Of these processes it is worth mentioning in a little more detail the 

 great capacity possessed by the human body for adjusting its salt loss to the supply 

 (Weiner & van Heyningen, 1952). It was first noted by Dill and his colleagues that 

 sweat of people living in the desert became progressively reduced in its salt content. 

 This is now known to happen only when salt intake is initially rather low. The kid- 

 ney in such circumstances immediately cuts down its salt concentration and output 

 and this is followed in a few days by a similar reduction in salt composition of 

 sweat. The normal sweat gland is in fact able to do considerable osmotic work in 

 producing a hypotonic fluid though this falls as high rates are approached. Many in- 

 dividuals can thus, after a period of adjustment, subsist in hot conditions on a 

 moderately low salt intake. There is however evidence of great individual variation 

 in this respect and the process may be attended by undesirable symptoms and a 

 lowered capacity for work. Salt imbalance is probably one of the commonest causes 

 for upset in hot conditions before acclimatization asserts itself. The evidence favours 

 the additional consumption of salt when sweating increases and water intake is cor- 

 respondingly high. 



Sweating and Acclimatization Acclimatization proper to hot desert conditions 

 shows itself in an increased capacity to perform muscular work and a concomitant 

 improvement in bodily heat regulation, as shown by a progressive reduction in the 

 pyrexia induced by high heat loads. There is a concomitant increase in the sensi- 

 tivity io heat stimulation of the sweat glands as shown both by some increase in 

 sweat production for a standard heat load and by a more rapid response. The acclima- 

 tized man accumulates less heat and must therefore handle a greater heat loss, or to 

 put it another way, he purchases reduced discomfort and high efficiency by a lower 



196 



