OSMOTIC PRESSURE OF URINE. 651 



of osmotic pressures by Pfeffer's method a determination of the freezing- 

 point of the solution. As van t' Hoff has shown, if A is the depression of 

 freezing-point and T the absolute freezing-point of the solvent (i.e., for 

 water, 273, and w the latent heat of fusion of ice = 79 cal.), then the 

 work A can be reckoned from the following formula : 



dA = - x dv. 



Thus for 1 per cent, solution of cane-sugar (A = -055) 

 ^ = -05579 X(fo . 



^ / O 



To reduce this result to gravitation units we must multiply by 424, 

 and we thus find that to separate the volume dv of pure water as ice 

 from 1 per cent, cane-sugar solution, a force is necessary equal to the 



055x79x424 

 pressure of a column of water of - TQ ~ metres in height. 



A depression of A= 1 corresponds therefore to an osmotic 



79 x424 

 pressure of 970"* ? that is to say, to 1227 metres of water. We 



have therefore to multiply A by 1227, in order to obtain the osmotic 

 pressure in metres of water of any solution. 



Now it is evident that, according to Ludwig's hypothesis, the osmotic 

 pressure of the urine might attain to but could never exceed that of 

 the blood plasma. On estimating the osmotic pressures of these two 

 fluids, we find that, under normal circumstances, the osmotic pressure of 

 the urine is considerably greater than that of the blood, so that work 

 must have been done in the separation of this concentrated fluid from 

 the more dilute blood plasma. Dreser 1 has estimated this work in a 

 case in which, during one night, 200 c.c. of urine were secreted with 

 A = 2'3. This was separated by the kidneys from the blood with 

 A = -56. In the production of this fluid Dreser finds that the work 

 done by the kidney amounts to 3 7 '03 7 kilogramme metres. This figure 

 by no means represents the maximum force which can be exerted by the 

 kidney. From a cat which had been deprived of water for three days, 

 Dreser drew off some urine with A = 472 C. The blood at the same 

 time had an osmotic pressure corresponding to A=0'66 C. These 

 differences in freezing point denote an osmotic difference of 498 metres 

 water, i.e. a pressure of 49,800 grins, per square centimetre. If this 

 work of concentration were carried out by the cells of the tubules, 

 these results would imply that these cells can exert a force six times 

 greater than the absolute force of human muscle (8000 grms. per square 

 centimetre). 



Assuming that the whole work of the tubules is confined to the act 

 of concentration, Dreser seeks, moreover, to demonstrate that the 

 glomerular secretion also involves the activity of living cells. Since 

 the blood pressure of 200 mm. Hg = 272 metres water, and A 1'0 C. = 

 1227 metres water, the highest possible difference between dilute urine 

 and blood, assuming that no concentration had taken place, could only 

 be A = 0'022 C. Dreser finds, however, that after beer drinking, and 



1 Arch.f. exper. Path. u. PharmakoL, Leipzig, 1892, Bd. xxix. S. 307. 



