700 THE SECRETION OF URINE 



tissue cedematous. The interpretation of these results is of 

 importance. 



Ludwig, in accordance with his view, pointed out that the 

 filtering force in the glomerulus must be the difference between 

 the capillary pressure, P, and the pressure of urine in the tubule, p. 

 Since p under normal circumstances is probably zero, P-p will 

 be equal to the capillary blood pressure. After section of the 

 spinal cord it was found that the secretion of urine ceased when 

 the arterial pressure had fallen to about 40 or 50 mm. Hg. He 

 considered, therefore, that the filtering force must be greater than 

 about 40 mm. Hg in order to be effective, a point to which we 

 shall have to return. But it was clear that, when P remained 

 normal, the filtering force might be reduced below its effective 

 limit equally well by increasing the value of p. He explained the 

 stationary level of the manometer in an obstructed ureter as the 

 point at which filtration ceased, because P-p was reduced below 

 its effective level. In other words, during obstruction the ureter 

 pressure represents the filtration pressure of the kidney, and must 

 therefore bear a constant relation to the arterial blood pressure. 

 Starling has given an experiment to show this relation. He 

 injected diuretin into a dog, and thereby raised the arterial blood 

 pressure from 115 to about 130 mm. Hg, and at the same time 

 the ureter pressure to about 90 mm. Hg. Gottlieb and Magnus 

 have, however, been unable to confirm this constant relation 

 between the arterial and ureter pressures. They find that diuretics 

 generally, and especially the salines, increase the ureter pressure, 

 but quite independently of the general blood pressure, so that the 

 actual difference between the two pressures may be either de- 

 creased or increased ; and that, as the diuresis passes off, there is 

 a fall of ureter pressure, again independent of the general arterial 

 blood pressure. They use this as an argument against the filtra- 

 tion theory. But, as Sollmann has pointed out, they overlooked 

 the fact that the hydrsemia, by reducing the viscosity of the 

 blood, would increase the glomerular, and so the ureter, pressure 

 without raising the general blood pressure ; and he has demon- 

 strated on excised kidneys that reduction of the viscosity of the 

 blood does raise the ureter pressure independently of blood 

 pressure. 



Heidenhain gave a different explanation of the effect of 

 obstructing the ureter. He observed the same phenomenon when 





