698 THE SECRETION OF URINE 



correspondingly decreases the rate of blood-flow through the organ. 

 It would appear, therefore, to be a means which might help to 

 decide between these two views. Complete obstruction of the 

 vein necessarily stops the blood-flow through the kidney, and 

 most observers have found that it has the same effect upon the 

 secretion of urine, although it is supposed to increase greatly the 

 pressure in the glomerular capillaries. Ludwig showed micro- 

 scopically that the interlobular veins became so swollen as to 

 obstruct mechanically the loops of Henle, and consequently the 

 passage of fluid down the tubules. Heidenhain did not accept 

 this explanation, and laid great stress on the experiment as show- 

 ing that it is the rate of blood-flow through the capillaries rather 

 than the pressure in them which determines the rate of secretion 

 of urine. This conclusion of Heidenhain's was certainly not justi- 

 fied, for he never disproved the truth of Ludwig' s observation, and 

 further, it is obvious that complete cessation of blood-flow through 

 the glomeruli must lead to such a concentration of the blood in 

 their capillaries that filtration would soon become impossible. 

 Since then, however, it has been shown by several observers that 

 partial obstruction of the vein reduces the flow of urine, and 

 De Sousa has measured the blood-flow through the kidney under 

 these circumstances. He found that, other things being equal, 

 the amount of urine secreted is directly proportional to the velocity 

 of blood-flow through the kidneys, and that every degree of venous 

 obstruction diminishes the secretion by slowing the blood-flow, 

 and quite apart from any change in the aortic blood pressure. He 

 was further able to disprove the statement of Schwarz that the 

 cessation of urinary secretion is due, not to the venous obstruction 

 as such, but to intravascular clotting, and that if a dog's blood 

 is made uncoagulable by previous defibrination, obstruction of the 

 vein then causes an increased secretion of urine. 



We may safely conclude that obstruction of the renal vein, 

 however slight, causes corresponding reductions in the flow of 

 urine and the rate of blood-flow through the kidney. But, before 

 accepting this as a powerful argument against Ludwig's view, it 

 is necessary to consider the effect of partial venous obstruction 

 on the filtering force, i.e. the difference between the pressures on 

 the two sides of the filtering membrane. It is fair to assume that 

 the capillary blood pressure would be raised ; the only uncertainty 

 on this point would be due to the fact that between the renal vein 



