CHEMISTRY OF DIGESTION AND NUTRITION. 275 



of proteid will yield ^ gram of urea, as may be calculated easily from the 

 amount of nitrogen contained in each. Since, however, some of the nitrogen 

 of proteid is eliminated in other forms uric acid, creatinin, etc. even an 

 exact determination of all the urea would not be sufficient to determine with 

 accuracy the total amount of proteid broken down. This fact is arrived at 

 more perfectly, as we shall explain later, by a determination of the total 

 nitrogen of the urine and other excretions. In addition to the urine, urea is 

 found in slight quantities in other secretions, in milk (in traces), and in sweat. 

 In the latter liquid the quantity of urea in twenty-four hours may be quite 

 appreciable as much, for instance, as 0.8 gram although such a large amount 

 is found only after active exercise. It has been ascertained definitely that urea 

 is not formed by the kidneys : it is brought to the kidneys in the blood for 

 elimination, the cells of the convoluted tubules being especially adapted for 

 taking up this material and transmitting it through their substance to the 

 lumen of the tubules. That urea is not made in the kidneys is demonstrated 

 by such facts as these: If blood, on the one hand, is irrigated through an 

 isolated kidney, no urea is formed, even though substances (such as ammonium, 

 carbonate) from which urea is readily produced are added to the blood ; on the 

 other hand, urea is constantly present in the blood (0.0348 to 0.1529 per cent.), 

 and if the two kidneys are removed, it continues to accumulate steadily in the 

 blood as long as the animal survives. It has been ascertained that the urea is 

 produced mainly in the liver ; an account of some of the experiments demon- 

 strating this fact is given on page 271. The most important questions that 

 remain to be decided are, Through what steps is the proteid molecule metab- 

 olized to the form of urea? and, What is the antecedent substance brought 



* O 



to the liver, from which it makes urea? It is impossible to answer these 

 questions perfectly, but recent investigations have thrown a great deal of light 

 on the whole process, and they give hope that before long the entire history 

 of the derivation of urea from proteids and albuminoids will be known. The 

 results of this work may be stated briefly as follows : 



1. Urea arises from proteids by a process of hydrolysis and oxidation, with 

 the formation eventually of ammonia compounds, most probably the ammo- 

 nium salt of carbamic acid, which are then conveyed to the liver and there 

 changed to urea. The latter part of this theory that the liver may produce 

 urea from carbamate of ammonia rests upon solid experimental evidence, as 

 follows : In the first place, Drechsel found carbamic acid in the blood of dogs, 

 and Drechsel and Abel have shown that it occurs normally in the urine of 

 horses as calcium carbamate; and Abel has recently shown that it may be 

 found in the urine of dogs or infants after the use of lime-water. Drechsel 

 has shown, further, that ammonium carbamate may be eon verted into urea. 

 If one compares the formulas of ammonium carbamate and urea, it is seen that 

 the former may pass over into the latter by the loss of a molecule of water, as 



Ammonium carbamate. Urea. 



