212 THE BODY AT WORK 



As carbonic acid and water they bear them to the lungs. But 

 nitrogen clings to hydrogen. Oxygen cannot detach its grasp. 

 Out of the molecule of albumin this firmly united couple slips, 

 without contributing anything to the energy which moves the 

 body and keeps it warm. Nitrogen is not a source of energy. 

 It even saves a portion of the hydrogen of albumin from com- 

 bustion. Urea burnt in a calorimeter has a balance of energy 

 to give up. 



Many attempts have been made to ascertain the stages 

 through which proteins pass on their road to urea. The search 

 for intermediate compounds is probably futile, since there is 

 no sufficient reason for supposing that proteins disintegrate in 

 stages, each a step less complex than the food and a step nearer 

 to urea. Every nitrogenous extractive found in the tissues is, 

 of course, on its road to urea. It will be removed as urea, 

 unless indeed, like uric acid or creatinin, it has to be excreted 

 without further change. But it appears to be impossible to 

 discover in the tissues any nitrogenous compounds which occur 

 in sufficient quantity to justify us in regarding them as inevit- 

 able halting-places on the downward road (cf. p. 146). 



The metabolism of albuminous substances, like other oxida- 

 tions, takes place chiefly in muscles. Very little is known 

 regarding the nature of the products. Urea is not amongst 

 them. Whatever they may be (cf. p. 267), they are carried to 

 the liver, in which they are turned into urea. 



The metabolism of the body is not normally derived from the 

 oxidation of nitrogenous foods. Failing a sufficient supply of 

 other kinds of food, they may be used as sources of energy ; 

 but we must picture them as splitting into carbonaceous and 

 nitrogenous portions. If, after the reserve of glycogen in the 

 liver has been brought low by abstention from carbohydrates 

 and fats, nitrogenous food is consumed, and the muscles are 

 then called upon to do severe work, the amount of carbonic 

 acid and water given off rises at once. The excess of urea 

 derived from the nitrogenous food which was destroyed for 

 the purpose of liberating the energy which the muscles ex- 

 pended makes its appearance some time later. If the diet 

 contains a sufficiency of carbohydrates, muscular work does 

 not increase urea. The output of urea is exceedingly steady. 

 It is not increased by muscular work, nor diminished, beyond 



