86 VERTEBRATE RESPIRATION 



(c) TEMPERATURE CONTROL 



In Chapter 6 we saw that warm-blooded or homoiothermic 

 animals (birds and mammals) maintain body temperatures which 

 are independent of the environmental temperature. This con- 

 trasts with poikilotherms, which are far less able to protect 

 themselves against thermal stresses and do so mainly by escap- 

 ing from them or by other forms of behaviour. The temperature 

 of a mammal does vary, however, and cannot be given a fixed 

 value. Not only does it vary from time to time but also in 

 different parts of the body. Thus in man 98-4° F. and 99-4° F.* 

 are often considered the normal oral and rectal tempera- 

 tures, but in fact they may range from 98-99° and 99-100° 

 respectively. These variations may result from feeding and other 

 causes. Furthermore there is a normal diurnal rhythm, the 

 lowest temperatures being recorded between 6 and 7 a.m. and 

 the highest some twelve hours later. The maintenance of this 

 more or less constant body temperature is the result of the inter- 

 action of processes concerned with heat loss and heat produc- 

 tion. There is an environmental temperature at which both of 

 these problems are most economically accomplished which for 

 man is around 86° F. At this temperature he is able to lose to 

 the environment all the heat formed by his basal metabolism 

 and retain only that which is necessary to maintain his normal 

 body heat, and there is no need to call upon reserve mechanisms 

 of heat loss. The temperature is a little high for his comfortable 

 every-day life, however, for he cannot easily dispose of any extra 

 heat produced during muscular exercise or assimilation of food. 



(i) Heat production and conservation 



The mechanisms of heat production involve the heat liberated 

 from chemical processes within the body, many of which take 

 place within the liver, but they are most readily controlled by 

 adjustments of muscular activity. Voluntary exercise such as 

 stamping the feet, swinging the arms, etc. is one method, but 



* 36-9" C. and 37-4° C 



