THE SECRETION OF URINE 699 



and the glomerulus is the capillary plexus round the tubules. On 

 the other, 1 i.e. the tubule, side of the glomerular epithelium it is 

 equally possible that the pressure would be increased and the 

 available filtering force thereby reduced. For, if Ludwig's obser- 

 vations are true and they have never been contradicted that 

 obstruction of the renal vein begins to compress the tubule close 

 to the glomerulus, then a rising pressure of glomerulus filtrate 

 within the tubule becomes possible. Sollmann has demonstrated 

 that this does take place in excised kidneys when perfused with 

 1 per cent, sodium chloride solution. Here the " urine " is a 

 mechanical filtrate, and the fact that every degree of obstruction 

 of the renal vein diminishes correspondingly the outflow of " urine " 

 can be explained only by decreased nitration. 



An a priori objective raised by Heidenhain to Ludwig's theory 

 may be dealt with here. He calculated from the probable per- 

 centage of urea in human blood that, in order to account for the 

 urea in a whole day's urine, 70 kilos of fluid would have to be 

 filtered through the glomeruli, of which about 68 kilos would 

 be reabsorbed. He estimated that the total circulation through 

 both kidneys in twenty-four hours amounted to only 130 kilos 

 of blood, and that it would therefore be necessary for the blood 

 as it passed through the glomeruli to lose more than half its 

 weight of fluid, a proposition which he considered absurd. De 

 Sousa has given a very different calculation as the result of his 

 experiments on the blood-flow through the kidneys. He assumes 

 that blood may contain O'l per cent. urea. In order that a day's 

 urine might contain 33 grm. of urea, 33 litres of fluid would have 

 to be filtered. He calculates that the blood-flow through both 

 kidneys together may amount to 882 litres in twenty-four hours. 

 33 litres is only 3' 7 per cent, of this amount ; consequently 

 Ludwig's view need entail a concentration of the blood as it passes 

 through the glomeruli by only 3' 7 per cent. Such a concentration 

 is a long way within the limits found by Barcroft to take place in 

 the submaxillary gland during activity. 



Obstruction of the Ureter. It has been shown that when a 

 mercury manometer is connected with the ureter above a com- 

 plete obstruction, the pressure rises at first rapidly to about 

 20 m.m. Hg, and then more slowly to about 60 m.m. Hg, at which 

 level it remains stationary. The ureter and pelvis -of the kidney 

 become distended with urine, and the kidney and surrounding 



