EXCRETION 459 



Urea. Urea is the most abundant of the organic constituents of the 

 urine and is present to the extent of from 2 to 3 per cent. It is a colorless 

 neutral substance, crystallizing under varying conditions in long silky needles 

 or in rhombic prisms. It is soluble in water and alcohol. It is composed of 

 CON 2 H 4 . When subjected to prolonged boiling, it combines with water, 

 giving rise to ammonium carbonate. The presence of Micrococcus urea 

 in urine will also convert the urea, by combining it with two molecules of 

 water, into ammonium carbonate, e.g., CON 2 H 4 + 2H 2 O = (NH 4 ) 2 CO 3 . 



The amount of urea excreted each day varies from 30 to 40 grams, the 

 average being about 34 grams and therefore represents an amount of 

 protein metabolized equivalent to from 90 to- 120 grams or an average 

 of about 100 grams. The remaining nitrogen-holding compounds in the 

 urine represent as shown by their nitrogen content a protein metabolism 

 of about 12 grams. As to how much of the urea or of the total nitrogen is 

 derived from the metabolism of tissue protein and how much from the 

 metabolism of the food protein that is not elaborated into tissue protein, 

 is difficult to state. It has been observed however in human beings in the 

 fasting condition that for a period of 10 days, there is a daily excretion 

 of about 21 grams of urea equivalent to about 63 grams of protein metabo- 

 lized. If it be accepted that approximately 63 grams of tissue protein are 

 metabolized each day then of the 34 grams of urea excreted, 13 grams must 

 come from about 40 grams of metabolized food protein. That the urea that 

 comes from the tissue protein is a rather constant factor and that the urea 

 that comes from the food protein is a variable factor is shown by the fact that 

 the amount of urea excreted rises and falls proportionately to the protein con- 

 sumed. As to the particular tissues that are undergoing protein metabolism 

 there is much obscurity. Contrary to what might be expected there is ap- 

 parently but little protein metabolism in muscle tissue for there is' no parallel- 

 ism between urea production and muscle work. Even after severe labor 

 extending over a period of some hours there is no noticeable increase in the 

 urea excreted. 



Seat of Formation and Antecedents of Urea. It has been stated in a 

 foregoing paragraph that the excretory organs are engaged in the process of 

 eliminating from the blood, rather than in elaborating, the end-products of 

 metabolism. Therefore the supposition is that the kidneys are not the seat 

 of urea formation but only the means by which it is eliminated from the 

 blood. This supposition is rendered highly probable from the following 

 facts: the blood of the renal artery contains from one-third to one-half 

 more urea than the blood of the renal vein; ligation of the renal arteries 

 or removal of the kidneys leads to an accumulation of urea in the blood to an 

 extent four times the normal amount in 24 hours; perfusion of the excised 

 kidney, which still retains its physiologic activity, with blood containing 

 known antecedents of urea is unattended with urea formation. These and 

 other facts of a similar character confirm the view that the kidney does not 

 manufacture but simply excretes urea brought to it by the blood. Since 

 urea is always present in the blood to an extent of from 0.04 per cent, to 

 0.06 per cent., i.e., from 4 to 6 grams per 10,000 grams, and that it is being ex- 

 creted at the rate of about 1.5 grams per hour, it is evident that it is being as 

 constantly formed in some one or more organs, and discharged into the 

 blood. 



