THE SECRETION OF URINE 



1143 



to a stimulation of the renal 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 ccm. 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 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 it 

 is possible that the epithelium of the tubules can secrete 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 composi- 

 tion of the blood or in its total volume. Water is absorbed from the ali- 

 mentary 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 determined entirely by the circulation through the kidney and 



