542 WASTE-PRODUCTS 



for amino-acids is limited. The excess of amino-acids absorbed from 

 the intestine is converted into urea by a series of steps which we are 

 about to discuss, and this is excreted promptly in the urine. On the 

 other hand the excretion of urea upon a diet low in proteins, but 

 abundant in fats and carbohydrates, may actually be less than in star- 

 vation, because the fats and carbohydrates spare the tissue-protein 

 from destruction for the production of the energy which is dissipated 

 by the body. 



The question of the region of the body in which urea originates has 

 been the subject of a great many investigations. Since it is so promi- 

 nent a constituent of urine, the kidneys naturally fall first under sus- 

 picion of being the organs in which the manufacture of this material 

 takes place. This possibility has been the subject of experimental 

 inquiry by a number of investigators. If the kidneys produced urea 

 to the extent of an important proportion of the total output, then 

 excision of the kidneys should lead to the disappearance of urea 

 from the body, or at any rate should not lead to its accumulation. 

 If, however, the kidneys simply eliminate urea which is produced 

 primarily by other organs, then excision of the kidneys should lead to 

 the accumulation of urea in the organs and tissue fluids. This- is what 

 actually occurs, and the accumulation of urea under these circumstances 

 and in conditions involving inefficient excretion by the kidneys, as in 

 Nephritis, has been repeatedly established. 



We must therefore look elsewhere than to the kidneys for the main 

 source of the urea which they excrete. Front a variety of different 

 experimental results we can definitely affirm that the Liver plays a 

 very important role in the production of urea; whether it is the exclus- 

 ive source of this substance or not cannot be regarded as definitely 

 established, but a very large proportion of the total output originates 

 in this organ. Thus if blood be perfused through the various organs 

 in such a manner that the same blood passes without renewal through 

 the tissues over and over again, no accumulation of urea in the blood 

 is noted in the case of the kidneys or of muscular tissues, but a very 

 pronounced accumulation occurs in the blood which is perfused through 

 the liver. 



The portal vein, which carries the blood containing absorbed food- 

 stuffs from the alimentary wall to the liver, runs parallel with, and 

 very close to, the inferior vena cava. By making an incision in the 

 adjoining sides of these veins and sewing the edges together, an oper- 

 ation which is known as Eck's Fistula, the portal circulation is short- 

 circuited and the blood from the intestine, with its load of food- 

 products, no longer passes through the tissues of the liver. The liver 

 is, however, still nourished by the circulation from the hepatic artery. 

 Animals upon which this operation has been performed will survive 

 for prolonged periods, and it was found by Pawlow and Nencki that in 

 such animals the urea excretion is greatly diminished while the ammonia 

 excretion is very considerably increased; in other words that ammonia 



