URINE 111 



state of equilibrium the quantity of urea excreted daily averages 33 

 crrammes (500 grains). The normal percentage in human urine is 

 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 proteids. The urea does not 

 come, however, direct from the food ; the food must be first assimi- 

 lated, and become part of the body before it can break down to form 

 urea. An exception to this rule is to be found in the case of the amido- 

 acids, especially leucine and arginine, which are formed in the intes- 

 tinal canal from proteids dm-ing digestion . These substances are carried 

 to the Hver and converted into urea ; but only a very small fraction of 

 the urea in the urine is formed in this way. Food increases the elimi- 

 nation of urea because it stimulates the tissues to increased activity ; 

 their waste nitrogenous products are converted into urea, which, 

 passing into the blood, is directly excreted by the kidneys. The 

 greater the amount of proteid food given, the more waste products do 

 the tissues discharge from then- protoplasm, in order to make room 

 for the new proteid which is built into its substance. 



Muscular exercise has httle immediate effect on the amount of 

 urea discharged. During intense muscular work there is a sUght im- 

 mediate increase of urea, but this is quite insignificant when compared 

 with the increase of work. This is strikingly different from what occxurs 

 in the case of carbonic acid ; the more the muscles work, the more 

 carbonic acid do they send into the venous blood, which is rapidly 

 discharged by the expired air. Careful research has, however, shown 

 that an increase of nitrogenous waste does occur on muscular exertion, 

 but appears as urea in the urine to only a sHght extent on the day of 

 the work ; the gi'eater part is excreted during the next day. 



Where is Urea formed?— The older authors considered that it 

 was formed in the kidneys, just as they also erroneously thought that 

 carbonic acid was formed in the lungs. Prevost and Dumas were the 

 first to show that after complete extirpation of the kidneys the forma- 

 tion of urea goes on, and that it accumulates in the blood and tissues. 

 Similarly, in those cases of disease in which the kidneys cease work, 

 urea is still formed and accumulates. This condition is called nramia 

 (or urea in the blood), and unless the urea be discharged from the 

 body the patient dies. There is no doubt, however, that it is not urea 

 but some antecedent of urea that acts most poisonously, and is the 

 cause of death, for considerable quantities of urea can be injected into 

 the circulation \Ndthout untoward results. 



Where, then, is the seat of urea formation ? Nitrogenous waste 



