166 INTERMEDIATE METABOLISM 



undergoing wear and tear, the amount of which must 

 necessarily be determined by activity and not by diet, the 

 nitrogen in the urine which originates in cell-breakdown 

 must be that part which is not influenced by diet — that 

 is, creatinine. This is " endogenous metabolism." 



The other constituents of urine — uric acid, ammonia and 

 " undetermined nitrogen " (which chiefly consists of amino- 

 acids and nitrogenous bases) — are partly of exogenous, 

 partly of endogenous origin. 



During starvation the amino-acid content of the blood 

 is shghtly increased. This is due to a breakdown of 

 protein in the less essential organs, such as the skeletal 

 muscles, and a transference of amino-acids to the indis- 

 pensable organs, such as the heart and brain. Migration 

 of amino-acid also occurs in fish during the spawning 

 season. Here the nucleo-protein of the sexual organs is 

 being built up at the expense of stored muscle protein 

 (see p. 172). 



The Formation of Urea 

 The amino group which is spht off from the amino-acid 

 in exogenous metabolism is converted into ammonia. This 

 is probably effected by a process of hydrolysis : — 



The ammonia thus hberated combines with any acid 

 radicles which may be present in the blood. Carbonic 

 acid being the most abundant of these, loose compounds 

 are formed — ammonium carbonate and ammonium car- 

 bamate. The close relation which these two substances 

 bear to one another and to urea is shown by their 

 formulae : — 



/ONH4 /ONH4 /NH2 



o=c< o=c< o=c< 



^0NH4 ^NHg ^NHg 



Ammonium carbonate. Ammonium carbamate. Urea. 



