174 PHYSIOLOGY FOR BEGINNKKS CHAP. 



but all the living tissues produce some heat, and they are 

 always producing it. When a tissue is in activity, as, for 

 instance, when a muscle is contracting or a gland is secreting, 

 it produces more heat than when it is at rest. When a man is 

 doing work his muscular tissues are oxidising faster, and more 

 heat is produced. 



When a man is neither gaining nor losing weight, the 

 energy he gives out daily must come ultimately from the 

 substances he is taking in. If we take the food a man eats 

 in a day we can determine experimentally the amount of 

 energy in the form of heat which this can give out when 

 it is oxidised to the same substances as those into which 

 it would be changed if it were built up into tissues and then 

 oxidised in the body, and we can compare this amount with the 

 amount of energy which the man produces in the day. The 

 energy which the oxidation of the food outside the body 

 produces is all heat; the energy which the man produces inside 

 his body leaves him partly as heat, partly as work done. 

 About one-sixth of the energy produced by oxidation in the 

 body is used to do work, and five-sixths appear as heat. When 

 we make this comparison we find that the amount of heat which 

 results from the oxidation of the daily food outside the body does 

 correspond to the heat (together with that portion of the energy 

 which is used to do work) which the man produces with it in 

 the day. The various food -stuffs vary in the amount of heat 

 they produce when oxidised, and this largely depends on their 

 composition. A given weight of fat will produce twice as much 

 heat as the same weight of carbohydrates or of proteids. This 

 is one reason why much fat is found by man more acceptable 

 and desirable as food in cold climates than in warm climates. 

 The fat must, however, be absorbed, assimilated, built up into 

 tissue before it is oxidised in the body, so that it is no use 

 taking a large amount of fat in order to produce much heat 

 unless this can be digested and assimilated. 



Distribution of Heat. The circulation of the blood dis- 

 tributes the heat, and keeps the temperature very nearly the 

 same throughout the body. When the blood is passing through 

 an organ which is at the time producing much heat, such as a 

 contracting muscle, it leaves the organ warmer than when it 

 entered it, and then circulating through the body distributes 



