1278 PHYSIOLOGY 



concentration, as indicated by its osmotic pressure and depression of 



freezing-point (A), approximate that of the blood-plasma. 



A dog received 40 grm. of dextrose dissolved in 40 cm. of water. 

 The following Table represents the relative concentrations of urine 

 and blood-serum at different stages in the diuresis thereby produced : 



A still closer approximation of the concentration of the urine to 

 that of the plasma was obtained by Galeotti in some experiments 

 in which the modifying influence of the tubular epithelium on the 

 glomerular transudate had been prevented by poisoning the animal 

 with corrosive sublimate, which causes destruction of the epithelium, 

 but is said to leave the glomeruli intact. 



Since the glomerular transudate must have a concentration 

 approximately identical with that of the blood-plasma, it would be 

 impossible for a urine formed by mere filtration to have a concentration 

 less than that of the blood-plasma. It is, however, of frequent occur- 

 rence that, after copious potations of tea or light beer, urine is passed 

 with an osmotic pressure and a molecular concentration considerably 

 below that of the blood. In one case Dreser obtained a urine with 

 a freezing-point of A = 0-16 C., and the same result has been 

 obtained on one or two occasions when the diuresis has been produced 

 by the administration of caffeine. If we assume that this hypotonic 

 fluid is formed by the glomeruli, we must at once give up any idea 

 of the process in these structures being essentially one of filtration. 

 But there is evidence that the epithelium of the tubules can secre(e 

 water as well as solid constituents. The fine adaptation of the kidney 

 to slight changes in the composition of the blood is apparently an 

 endowment of the tubular epithelium, and in those cases where large 

 quantities of hypotonic urine are passed there is not at any time any 

 appreciable change either in the composition of the blood or in its 

 total volume. Water is absorbed from the alimentary canal and is 

 almost immediately excreted by the kidneys. When we attempt to 

 produce the same effect by infusion of large quantities of water or 

 hypotonic solutions into the blood-stream, we get a flow of urine 



