CHEMISTRY OF DIGESTION AND NUTRITION. 335 



as an amide of carbonic acid, having therefore the structural formula of 



C0< y„- It occurs in the urine in relatively large quantities (2 per cent. -f-). 



As the total quantity of urine secreted in twenty-four hours by an adult male 

 may he placed at from 1500 to 1700 cubic centimeters, it follows that from 30 

 to 34 grams of urea are eliminated from the body during this period It is 

 the most important of the nitrogenous excreta of the body, the end-product 

 of the physiological oxidation of the proteids of the body, and also of the 

 albuminoids when they appear in the food. If we know how much urea is 

 secreted in a given period, we know approximately how much proteid has 

 been broken down in the body in the same time. In round numbers, 1 gram 

 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 (0.03 18 to 0.1529 per cent.), 

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

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

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

 strating this fact is given on page 331. 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 

 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, which are then conveyed 

 to the liver and there changed to urea. Drechsel lias suggested that am- 

 monium carbamate tonus one at least of the ammonia compounds that arc con- 



