512 THE HUMAN BODY 



by inducing a profuse perspiration with resulting loss of water. 

 The intense thirst which follows the exercise leads to abundant 

 ingestion of water and a speedy restoration of the lost weight. 



Nitrogen Equilibrium. Those metabolic activities of living 

 tissues which result in tissue break-down are particularly associ- 

 ated with the use of protein foods, since, as we have seen, their 

 repair can be accomplished only by proteins. The characteristic 

 constituent of protein is nitrogen; and the simplest way to esti- 

 mate the amount of protein contained in any food mass, or repre- 

 sented by any particular amount of excretion, is to determine the 

 nitrogen and multiply the weight of it present by 6.25, the fraction 

 of protein which is nitrogen. We shall learn in the chapter on 

 Excretion (Chap. XXXI), that in the healthy Body an accumula- 

 tion of nitrogen-containing excretory products never occurs; as 

 fast as wastes are formed they are gotten rid of. It follows, 

 then, that if there is less nitrogen being given off than taken in, 

 the living tissues of the Body must be increasing in amount, and 

 if more is given off than is obtained in the food the living tissues 

 must be wasting away. In the healthy adult Body, neither of 

 these conditions is at all usual; the intake and outgo of nitrogen 

 balance each other and the Body is in nitrogen equilibrium. 



It has been chiefly through experimental studies of nitrogen 

 equilibrium that our ideas of the twofold function of protein, as 

 tissue-restorer and as fuel, have been gained. If an animal be 

 fed large enough quantities of protein he requires no other food, 

 and if healthy maintains nitrogen equilibrium upon this high level, 

 the large nitrogen intake being exactly balanced by an equally 

 large outgo. Now by substituting other foods, as carbohydrates 

 or fats, for part of the protein, the nitrogen intake and outgo are 

 each less in quantity, but they still balance; the animal is in nitro- 

 gen equilibrium upon a lower level. If the substitution of other 

 foods for protein is increased a point is presently reached when 

 the nitrogen outgo exceeds its intake; the animal is not getting 

 enough protein for his needs, and so his own tissues are breaking 

 down (p. 501). During the growth period, on the other hand, or 

 after a wasting illness, when new tissue is being formed, the nitro- 

 gen balance is the other way; the amount of balance lost from the 

 Body daily is less than that consumed. The hearty appetites of 

 children and convalescents are associated with this necessity of 





