EXCEETIONS 489 



generally some reduction in the amount of urea in the urine, and an 

 increase in the ammonia content. 



Urea is an extremely soluble and relatively non-toxic substance. 

 These two properties have a particular significance in view of the fact 

 that urea is the chief end product of protein metabolism, and is almost 

 wholly eliminated through the kidney, the portion excreted through other 

 channels such as the skin being relatively unimportant. The quantitative 

 output of urea is closely proportional to the amount of protein ingested. 

 Variations of 10 to 40 grams may be encountered in perfectly normal indi- 

 viduals. The percentage of urea is dependent upon the volume of urine 

 in addition to the protein of the diet, and when it is considered that the 

 former may vary from 500 to 2000 c.c. it is evident that but little informa- 

 tion concerning the quantity eliminated can be gained from a knowledge 

 of merely the percentage of urea. The urea nitrogen in ^proportion to 

 the total nitrogen excreted may likewise be greatly influenced by the 

 amount of protein in the diet. Thus with a high protein intake, the urea 

 nitrogen may make up as much as 90 per cent of the total nitrogen; 

 while with a diet containing relatively little protein but considerable 

 carbohydrate and fat, the proportion may be as low as 60 per cent. (See 

 table on p. 486.) With a nitrogen intake of 20 grams the urine would 

 contain approximately 18 grams of nitrogen, of which about 16 grams 

 would be in the form of urea ; whereas with a nitrogen intake of 7 grams 

 the excretion of urea nitrogen may be as low as 4 grams. An average 

 quantitative output of urea with its nitrogen equivalent and the relation 

 of the latter to the total nitrogen output is given in the table on page 

 487. It will be readily seen that it is quite essential in considering 

 the excretion of total nitrogen and urea to compare these values with 

 the nitrogen of the food, because only when the nitrogen output is out 

 of proportion to the intake can an abnormal condition be presumed to 

 exist. 



When the rate of metabolism is accelerated as in fevers, exophthalmic 

 goiter, etc., or by, the consumption of large amounts of protein as in 

 diabetes, the total nitrogen and urea may be greatly augmented. 

 Although the function of excreting urea may be much impaired in 

 nephritis a recognition of this fact simply from the output of urea is 

 difficult. Information in this regard may be more readily secured from- 

 an analysis of the blood. 



Ammonia. Under ordinary conditions the nitrogen of ammonia, in 

 combination with urinary acids, is present in the urine to the extent of 

 2.5 to 4.5 per cent of the total nitrogen eliminated, i.e., about 0.5 gram per 

 day. A considerable portion of this probably represents urea which has 

 been reconverted into ammonia so that it might be utilized to neutralize the 

 sulphuric, phosphoric, uric acid, etc., formed in the process of normal me- 

 tabolism or introduced with the food. This procedure probably operates to 



