438 THE HUMAN BODY. 



with some water, quite independently of any formation of 

 transudation urine. In mammalia we cannot separate the 

 glomerular secretion from the tubular as in amphibia; and 

 the diuresis which administration of urea causes in them is 

 in part due to increased glomerular activity, as urea dilates 

 the kidney vessels and causes more blood flow through the 

 glomeruli, which causes the transudation of more water 

 through them; but the simultaneous increase of urea is 

 almost certainly due to special activity of the other parts of 

 the tubules. 



The proteids and albuminoids of food may while within 

 the organism have been built up into tissue or may have 

 remained in solution in the liquids; but in either case they 

 are sooner or later broken up and oxidized, the main final 

 products being carbon dioxide, water, and urea. But this 

 breaking down may and does occur in many stages and by 

 different modes in the various tissues; and there is no doubt 

 that most of the intermediate processes in the chemical 

 degradation of albuminous compounds take place outside 

 the kidneys. It was, however, at one time believed that the 

 urea itself was a kidney product : that the penultimate ni- 

 trogenous products of proteid degradation were brought to 

 the kidneys, and that there the final formation of urea took 

 place. But if this were so there could not be less urea in the 

 blood leaving the kidneys by their veins than in that brought 

 to them by the renal arteries; yet such is the case. And 

 further, if urea be made in the kidneys it ought not to 

 accumulate in the blood of animals from whom both kidneys 

 have been removed, as it is now known to do, though not 

 the immediate cause of the symptoms of so-called urcemic 

 poisoning seen in persons with extensive kidney disease. 

 So far, then, as urea is concerned the cells of the kidney 

 tubules are not its producers; they have a special affinity for 

 it and pick it up from the lymph of the kidney, which in 

 turn gets it from the blood. The cells then pass it on with 

 some water, and no doubt other things, into the tubules 

 which they line. That it is the epithelial cells of the 

 contorted portions of the tubules which especially exer- 

 cise this selective power is, so far as urea is concerned, 

 a presumption based on their histological characters, but 

 there is evidence that these cells have a special selective 

 power for some other substances circulating in the blood. 



