THE SECRETION OF URINE 



1191 



epithelium, the differences between blood plasma and urine should be 

 greatest at the height of the diuresis, when the concentration of the specific 

 stimulant is also at its highest. The following experiment shows that the 

 more rapid the secretion of urine, the more closely does its 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 c.c. 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 int.act. 



Since the glomerular transudate must have a concentration approxi- 

 mately 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 occurrence 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 

 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 apparently deter- 

 mined entirely by the circulation through the kidney and having a con- 

 centration not inferior to that of the blood. The passage of hypotonic urine 



