FOOD DIGESTION AND RESPIRATION 63 



it. But the nitrogenous part of the protein used for energy purposes 

 is not completely oxidised. Urea, when burned with oxygen, gives 

 a certain, not very large, amount of energy, but it must be allowed 

 for in calculating the energy value of proteins. 



The unit of heat in which the energy value of food is expressed 

 is the large calorie, in physiological discussion usually spoken of as 

 the calorie simply. This is the amount of heat required to raise 

 the temperature of one kilogram of water by one degree centigrade, 

 or more precisely, to raise its temperature from o to i, since the 

 specific heat of water varies somewhat at different temperatures. 



The performance of a given amount of work by our muscles 

 requires the provision of its equivalent quantity of energy in food. 

 This is obvious as regards work done on objects in the outer world. 

 But a living organism differs from a machine which consumes no 

 energy when at rest, in that the heart must go on, breathing must 

 go on, and some other functions require energy even when the 

 body appears to be at rest. It may rather be compared to such a 

 machine as a circular saw, which is kept running when not actually 

 occupied in sawing wood. A certain quantity of energy is used in 

 friction and in fanning the air, but the consumption is greatly 

 increased when a beam of wood is sawn. 



The consumption of energy for the internal needs of the 

 organism is known as the " basal metabolism" or basal consumption. 

 This is the first time that we have used the word " metabolism," 

 and it needs definition. It is simply a convenient word to express, 

 briefly, the series of chemical changes undergone by chemical 

 compounds in the organism. Thus the metabolism of carbohydrate 

 means the various stages through which it passes before final 

 oxidation to carbon dioxide and water. 



The actual amount of energy required for basal needs varies, 

 naturally, with the size of the organism. It is almost exactly one 

 calorie per hour for each kilogram weight in man when asleep ; so 

 that, for a man of average weight (70 kilos or 1 1 stone), it amounts 

 to 1700 calories per day. The amount required when external 

 work is done differs according to the amount of this work, but the 

 following table shows the approximate accepted values : 



Basal, in sleep - - 1,700 calories. 



Do. awake, but at rest - 2,100 



Sedentary occupation - - 2,500 



Light work - - 3,000 



Moderate work - - 3,500 



Heavy work - - 4,000 to 9 ooo or more calories. 



Carbohydrate alone might give the energy required ; but we 

 have seen that protein is necessary to afford nitrogen for replace- 

 ment of wear and tear, and since it also gives energy, a diet might 



