ANIMAL HEAT 179 



Proteids are not completely oxidized in the body, for the 

 urea formed from it can still undergo oxidation. If the heat 

 value of urea is subtracted from that of proteid, there remain 

 for one gram of proteid 4. 1 calories. 



The physiological heat values are : 



For i g proteid 4.1 cal. ; I g fat 9.3 cal. ; I g carbo- 

 hydrate 4. I cal. 



As far as their heat production for the organism is con- 

 cerned, the following substances are isodynamic : 2.3 g 

 proteid (or gelatin) = I g fat = 2.3 carbohydrate. 



If the extent of metabolism is known, we can calculate 

 from the heat value of the substances oxidized the amount 

 of heat formed. Conversely, by finding the amount of heat 

 produced we can calculate the extent of metabolism ; but 

 this calculation is not conclusive as to the individual kinds 

 of foodstuffs used. 



The production of heat is measured by the water- or air-calo- 

 rimeter. In the water- calorimeter the body is placed in a tin 

 case which is surrounded by a layer of water. The heat given off 

 by the animal heats this water. The respiratory air is supplied 

 through tubes of which the one carrying the exhaled air passes 

 through the layer of water which surrounds the case, so that the 

 heat of the exhaled air is imparted to the water also. From the 

 increase in temperature of the water, the amount of heat lost by 

 the body can be calculated. This amount equals the heat formed, 

 for the body temperature is the same at the end as at the beginning 

 of the experiment. In the air-calorimeter the tin case is sur- 

 rounded by a layer of air whose expansion by the heat measures 

 the amount of heat set free. 



The adult resting human being produces in twenty-four 

 hours about 2400 calories, or in one hour 100 calories. 

 This is 34 calories per kilogram of body weight in twenty- 

 four hours, and 1.4 cal. in one hour. 



The amount of heat produced is dependent upon the same 

 circumstances as metabolism. Muscle activity increases 

 .heat production, for, of the extra energy set free thereby, 

 only a part can be used in the performance of work, the rest 

 being changed to heat. Of all the energy set free by ;i 

 working body, at most only one-fourth can be utilized for 



