574 METABOLISM, NUTRITION AND DIETETICS 



observations in man), and the average quantity of blood in them at 

 rather less than one-fourth of their weight, or 35 grammes, and con- 

 sider 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 excre- 

 tion 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 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 is 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. 



f Where, then, is urea chiefly formed ? The answer to this question 

 is that, while some urea is probably produced from amino-acids in 

 all the tissues, one oman is particularly associated with this function 

 namely, the liver.f 



There is no reason to suppose that the hepatic cells, so far as the 

 repair of their own protoplasm or the supply of energy for their own 

 special work is concerned, require to metabolize particularly large 

 quantities of amino-acids as compared, for instance, with the 

 muscles. Glycin, however, they must have for the manufacture of 

 glycocholic, and cystin for the manufacture of the taurin of taurocholic 

 acid. In addition, the liver is known to possess the power of utilizing 

 amino-acids for the formation of dextrose and eventually of glyco- 

 gen, and a portion of the surplus amino-acids of the food may be 

 withdrawn from the blood of the portal vein for this purpose, just 

 as the surplus of dextrose is withdrawn. 



t The liver contains a relatively large amount of urea, and there is 

 strong evidence that it is the manufactory in which a great part of 

 the nitrogenous relics of broken-down proteins reach the final stage 

 of urea) This evidence may be summed up as follows: 



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

 carbonate mixed with the blood passed through its vessels, while 

 no urea is formed when blood containing ammonium carbonate is 

 sent through the kidney or through muscles. Other salts of am- 

 monium, such as the lactate, the formate, and the carbamate, under- 

 go a like transformation in the liver. It is difficult, in the light of 

 this experiment, to resist the conclusion that the increase in the 



