METABOLISM 9 



6,500 kgm., or 21 foot-tons, daily. Here, again, in 

 pathological conditions this necessary work may be 

 greatly increased in amount, and the daily output of 

 energy of the body proportionately raised. 



Of all the items which fall under the heading of 

 internal work, heat production is, however, by far the 

 greatest. The mere performance of work by the heart 

 and the muscles of respiration involves the liberation as 

 a by-product of more than twice as much energy in the 

 form of heat as is actually required for the performance 

 of the mechanical work of respiration and circulation. 

 Whether besides this^waste heat an additional quantity 

 is produced as a result of the mere vital activity of the 

 cells (apart from any work done) is a matter on which 

 physiologists are not all agreed, and we shall return to 

 its discussion later on (Chapter II.). The question is an 

 important one ; for if heat results from the mere ' living ' 

 of the cells if it be a by-product or excretion of life as 

 such then it is conceivable that cells of a low degree of 

 vitality may produce less heat, and therefore expend less 

 energy, than those which are more ' alive,' and an 

 explanation would be found for the apparently greater 

 degree of economy in metabolism which some individuals 

 exhibit when compared with others. Some such explana- 

 tion, as we shall see, is almost necessary to account for 

 individual differences in nutrition. 



(ii.) The item digestive work covers the expenditure 

 of energy required for the digestion and absorption of 

 the food. It is an item of no small importance, amount- 

 ing as it does in the case of an ordinary mixed diet to 

 about 150 or 200 Calories i.e., about 6 to 8 per cent, of 



