586 THE URINE [CH. XXXIX. 



nitrogenous equilibrium, taking daily 100 to 120 grammes of protein in 

 his food, the quantity of urea secreted daily is about 33 to 35 grammes 

 (500 grains). The percentage in human urine would then be 2 per 

 cent.; but this also varies, because the concentration of the urine 

 varies considerably in health. In dogs it may be 10 per cent. 

 The excretion of urea is usually at a maximum three hours after a 

 meal, especially after a meal rich in proteins. If, therefore, people 

 adopt the Chittenden diet, which contains about half the quantity of 

 protein which is present in the more usual Voit dietary, their urine 

 will naturally show a nitrogenous output of half of that which is now 

 regarded as normal. In those who adopt such a reduced diet, Folin 

 has shown that the decrease in urinary nitrogen falls mainly on the 

 urea fraction, and in some cases the urea excreted accounted for only 

 66 per cent, of the total nitrogen. The other nitrogenous katabolites 

 of the urine alter comparatively little under such circumstances, and 

 the creatinine in particular remains remarkably constant in amount. 



In our study of protein absorption (p. 545), we have already 

 indicated that the amino-acid fragments of the food-protein are 

 utilised in two ways. A small part is used by the tissue cells for 

 the reconstruction of their protein which has undergone katabolism. 

 In time this will in turn be katabolised, and the waste products 

 discharged as ammonia, creatinine, and a certain amount of urea. 

 This form of metabolism may be termed tissue or endogenous 

 metabolism, and its amount is constant and independent to a great 

 extent of the food. The other and larger part of the cleavage pro- 

 ducts of the food protein are not made use of thus, but are 

 deamidised, and the nitrogenous portion is converted into urea by 

 the liver, and discharged by the kidney. This part of metabolism 

 may be termed exogenous ; it is variable in amount, and depends on 

 the quantity of ingested protein. 



That the liver is the organ where urea is made is shown by the 

 following considerations : 



1. After removal of the liver in such animals as frogs, urea 

 formation almost ceases, and ammonia is found in the urine instead. 



2. In mammals, the extirpation of the liver is such a severe 

 operation that the animals do not live. But the liver of mammals 

 can be very largely thrown out of gear by connecting the portal vein 

 directly to the inferior vena cava (Eck's fistula). This experiment 

 has been done successfully in dogs ; the amount of urea in the urine 

 is lessened, and its place is taken by ammonia. 



3. When degenerative changes occur in the liver, as in cirrhosis 

 of that organ, the urea formed is much lessened, and its place is 

 taken by ammonia. In acute yellow atrophy urea is almost absent in 

 the urine, and, again, there is considerable increase in the ammonia. 

 In this disease ammo-acids such as leucine and tyrosine are also found 



