l8o HUMAN PHYSIOLOGY 



mechanical work ; the remaining, three-fourths is set free as 

 heat. During hard work an adult man produces in twenty- 

 four hours, for every kilogram of body weight, an amount 

 of heat, including the external work, equal to 55 calories. 



2. The loss of heat. The body continuously loses heat: 



(1) By radiation and conduction from the surface of the 

 body to the surrounding air, which, as a rule, is colder than 

 the body. 



(2) By the evaporation of water from the skin, especially 

 by the secretion of sweat. By this means the body can lose 

 heat when the surrounding medium has a higher temperature 

 than the body itself. 



(3) By exhaling air which has been heated to the body 

 temperature and is saturated with water vapor. The water 

 vapor is imparted to the expired air by the evaporation of 

 water from the mucous membranes of the air passages. 



(4) By heating up the ingested food and drink ; in other 

 words, by voiding excretions heated to the body tempera- 

 ture (urine, faeces). 



Of all the heat lost by the body, about 8o# is lost by 

 radiation, conduction, and evaporation from the skin; about 

 15$ by evaporation from the mucous lining of the air 

 passages ; one half of the rest by expired air, and the other 

 half by the excretions. 



The amount of heat which is lost in each of these ways is 

 variable. The lower the external temperature, the more 

 heat is lost by conduction from the skin and by heating the 

 inhaled air ; the loss of heat by evaporation is greater, the 

 drier the air and the greater the amount of sweat secreted. 

 The heat lost by expired air depends upon the frequency 

 and depth of respiration. 



3. Body temperature. Man belongs to the warm- 

 blooded or homoiothermic animals whose body temperature 

 is, apart from very slight variations, constant. The body 

 temperature of man is 36.537.5 C. 



The body temperature is measured by placing a thermometer in 

 the rectum, vagina, mouth, or axilla, the arm being placed in the 

 proper position around the thermometer. 



