182 HUMAN PHYSIOLOQY 



constant body temperature smaller animals must produce 

 more heat per kilogram of body weight than larger animals. 

 The adult human being produces, at rest, per kilo-hour 1.4 

 calories, while a child four years old produces about 2.5 

 calories, a rabbit 5.6 calories. 



If the loss of heat is stated in terms of the unit of body surface, 

 it is found that it is about the same in animals of various sizes. 

 The amount of heat lost by man per square meter is about 1200 

 calories in 24 hours. 



Variations in the loss of heat take place because of: 



(1) Increased or decreased supply of blood to the skin, 

 whereby the heat carried to the cooling body surface is in- 

 creased or decreased. The supply of blood to the skin is 

 increased by the dilation of the cutaneous vessels and the 

 increase of pulse rate ; it is decreased by the contraction of 

 the vessels and the decrease of the pulse rate. 



(2) Secretion of sweat, which, by evaporation, cools the 

 body. 



(3) Increase or decrease in the frequency or depth of the 

 respirations, whereby more or less heat is given off by the 

 expired air. 



Muscular activity, by which more heat is produced, or 

 raising of the external temperature (warm weather) are fol- 

 lowed by perspiration, increased pulse and respiration, and 

 dilation of the cutaneous vessels; lowering of the external 

 temperature (cold weather) causes constriction of the cuta- 

 neous vessels. 



We can voluntarily regulate the loss of heat by warming 

 ourselves, by clothing, by the position of the body, and by 

 partaking of cold or warm drinks. We can regulate the 

 production of heat by voluntary muscular activity. In 

 animals hairs and feathers serve to regulate the loss of heat. 



Our ability to keep the body temperature constant by 

 means of the heat-regulating mechanism is, however, limited. 

 This regulation of temperature fails when the temperature of 

 the surrounding medium is too high or too low, so that 

 changes in the production or loss of heat are no longer able 



