METABOLISM, NUTRITION AND DIETETICS 439 



the average difference in the percentage of urea in the blood 

 coming to and leaving them, we should at once be able to 

 decide whether the whole of the urea in the urine reaches the 

 kidneys ready made, or whether a portion of it is formed 

 by the renal tissue. Although data of this kind are as yet 

 inexact and incomplete, it is not difficult to see that all, or 

 most of the urea, may be simply separated by the kidney. 



If we take the weight of the kidneys of a dog of 35 kilos at 

 1 60 grammes (-jY^th of the body-weight is the mean result of a great 

 number of observations in man), and the average quantity of blood 

 in them at rather less than one-fourth of their weight, or 35 grammes, 

 and consider that this quantity of blood passes through them in the 

 average time required to complete the circulation from renal artery 

 to renal vein, or, say, ten seconds, we get about 300 kilos of blood as 

 the flow through the kidneys in twenty-four hours. At '3 per 1,000, 

 the urea in 300 kilos of blood would amount to 90 grammes. Now,. 

 Voit found that a dog of 35 kilos body-weight, on the minimum proteid 

 diet (450 to 500 grammes lean meat per day) which sufficed to main- 

 tain its weight, excreted 35 to 40 grammes urea in the twenty-four 

 hours. If, then, the renal epithelium separated somewhat less than 

 half of the 90 grammes urea offered to it in the circulating blood, the 

 whole excretion in the urine could be accounted for, and the blood 

 of the renal vein would still contain more than half as much urea 

 as that of the renal artery. So that the whole of the urea in the urine 

 may be simply separated by the kidney from the ready-made urea of 

 the blood. 



Another line of evidence leads us to the same conclusion :: 

 that the kidney is, at all events, not an important seat of 

 urea-formation. When both renal arteries are tied, or both 

 kidneys extirpated, in a dog, urea accumulates in the blood 

 and tissues ; and, upon the whole, as much urea seems to* 

 be formed during the first twenty-four hours of the short 

 period of life which remains to the animal as would under 

 normal circumstances have been excreted in the urine. 



Where, then, is urea chiefly formed? We should naturally 

 look first to the muscles, which contain three-fourths of the- 

 proteids of the body ; but we should look there in vain for 

 any great store of urea only a trace is normally present. 

 The liver contains a relatively large amount, and there 

 is very strong evidence that it is the manufactory in- 

 which the greater part of the nitrogenous relics of broken- 

 down proteids reach the final stage of urea. This evidence- 

 may be summed up as follows : 



(i) An excised 'surviving' liver forms urea from ammonium 



