500 A MANUAL OF PHYSIOLOGY 



the ultimate products, under the influence of oxygen, being carbon 

 dioxide, water, and comparatively simple nitrogen-containing sub- 

 stances, which after further changes appear in the urine as urea, 

 kreatinin, uric acid, and other bodies. We shall see later on that 

 a great part of the urea excreted does not arise in the decomposition 

 of living protein or protoplasm, but is the form in which ' surplus ' 

 nitrogen is eliminated in the preparation of the food-protein for 

 assimilation by the tissues. We have no definite information as 

 to the production of water from the hydrogen of the tissues, except 

 what can be theoretically deduced from the statistics of nutrition 

 (p. 540). A few words will be said a little farther on about the 

 production of carbon dioxide from proteins ; we have now to con- 

 sider the seat and manner of formation of the nitrogenous meta- 

 bolites. And since in man and the other mammals urea contains, 

 under ordinary conditions, by far the greater part of the excreted 

 nitrogen, it will be well to take it first. 



Formation of Urea. The starting-point of all inquiries into 

 the formation of urea is the fact that it occurs in the blood in 

 small amount (4 to 6 parts per 10,000 in man ; 3 to 15 parts per 

 10,000 in the dog), the largest quantity being found when the food 

 contains most protein and at the height of digestion, the smallest 

 quantity in hunger (Schondorff) . Evidently, then, some, at least, 

 of the urea excreted in the urine may be simply separated by the 

 kidney from the blood ; and analysis shows that this is actually 

 the case, for the blood of the renal vein is poorer in urea than that 

 of the renal artery, containing only one-third to one-half as much. 

 If we knew the exact quantity of blood passing through the 

 kidneys of an animal in twenty-four hours, and 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 (^lyth 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. Even at 0-3 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 

 protein diet (450 to 500 grammes of lean meat per day) which 

 sufficed to maintain its weight, excreted 35 to 40 grammes of 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 



