FORMATION OF UREA 141 



place along one path varies extremely, in a quantitative sense, 

 with the amount of protein in the food. The chief 'end products 

 formed are urea and inorganic sulphates. This form of kata- 

 bolism is not essentially connected with the life and nutrition 

 of living substance of the body as a whole, and is, therefore, 

 termed exogenous. The other form of protein metabolism 

 is practically constant in amount for any given individual, 

 and is independent of the quantity of protein in the food. 

 The characteristic end products formed are kreatinin and 

 neutral sulphur. Since it is an expression of the metabolism 

 of the living material of the body itself, it is termed endogenous 

 metabolism. 



The blood proteins, serum albumin and serum globulin, par- 

 ticularly the former, have been looked upon as a resynthesis 

 of part of the digestion products, and this function has been 

 attributed to the intestinal mucosa. If this is the case, then 

 the proteins must again be decomposed in the cells of the 

 various tissues in order that the amino-acids may be recombined 

 into tissue proteins. On the other hand, it is conceivable, 

 that only a moderate synthesis takes place in the intestinal 

 wall, sufficient for tissue needs, and that the blood proteins 

 are material proper to the circulating tissue, blood. They 

 might thus serve as a storehouse of protein material to be 

 drawn upon in case of starvation. In the latter condition it 

 is said that serum albumin is relatively decreased while serum 

 globulin is increased. They may, therefore, have a double 

 source the former being more closely related to food pro- 

 teins; the latter, to tissue proteins. Eventually, it is believed, 

 they as well as other more complex body proteins disintegrate 

 into comparatively simple nitrogen-containing substances 

 which appear in the urine as urea, uric acid, kreatinin, and 

 other bodies. 



Formation of Urea. Urea is found in the blood of man to 

 the extent of 0.04 to 0.06 per cent. Since urea is a waste sub- 

 stance and the kidney an excretory organ, it is a simple' matter 

 to conclude that the urea of the urine is separated from the 

 blood by the kidney. All evidence tends to show that the 

 kidney itself is a very unimportant seat of urea manufacture. 

 Muscle, likewise, though forming three-fourths of the proteins 

 of the body, contains only a trace of urea. The liver, on the 



