vin THE EXCEETION OF UEINE 435 



(a) To obtain the 35 grms. urea excreted daily in the urine, 

 no less than 70,000 c.c. of fluid must filter through the glomeruli 

 of the two kidneys, under the most favourable computation, 

 according to which urea would be present in a maximal amount 

 of 0*05 per cent. 



(&) Of this enormous quantity of filtrate, the no less enormous 

 amount of 68,000 c.c. must be reabsorbed by the uriniferous 

 tubules on the supposition that the day's urine is represented by 

 2000 c.c. 



The absurdity of these conclusions is obvious when we consider 

 that a man weighing 75 kilos, has about 6 kgrms. blood, which 

 performs some three circulations per minute. On Heidenhain's 

 computation not more than 130 kgrms. can pass through the 

 kidneys in 24 hours, more than half of which (70 kgrms.) has to 

 filter through the glomeruli ! 



Another still more serious objection to the mechanical theory 

 is presented by the concentration or osmotic pressure of the urine, 

 which is almost always higher than that of the blood and lymph 

 circulating in the kidneys. Admitting the second part of Ludwig's 

 theory, i.e. that the glomerular filtrate gradually concentrates along 

 the course of the tubules, because the lymph in which these are 

 bathed is more concentrated than the urine, so that reabsorption 

 of water takes place by endosmosis, it is obvious that this reabsorp- 

 tion must cease so soon as the urine becomes isotonic with the 

 lymph ; in other words, the urine might reach the concentration 

 and osmotic pressure of the lymph and blood, but could never 

 exceed it. The formation of urine with a higher concentration 

 than the blood cannot be explained as a simple physical pheno- 

 menon, but necessitates the intervention of an active participation 

 of the secreting cells. 



Dreser (1892) calculated the sum of this work for two special 

 cases. One urine secreted in a night to the amount of 200 c.c. 

 had A = 2-3 C., while the blood of the same person gave A = 0'56 C. 

 According to Dreser's calculation, the work performed by the 

 kidneys in the secretion of this urine must amount to 37'037 kilos. 

 In a cat prevented from drinking for three days, Dreser obtained 

 a urine in which A = 4-72 C., while the blood of the same animal 

 showed A = 0'66 C. This difference in the freezing-point corre- 

 sponds with a difference of osmotic pressure =498 mm. water, or 

 a pressure of 49,800 grms. per square cm. But it should be noted 

 that, according to the observations of v. Rhorer (1905) and 

 Galeotti (1907), this hypothetical value of the total osmotic work 

 of the kidney is in all probability much less than that which is 

 actually performed, assuming 'that the kidney functions by a 

 mechanism such as is assumed by chemists and physicists, i.e. as a 

 semi-permeable membrane. Supposing this concentration to be 

 performed osmotically by the] walls of the renal tubules, we should 



