548 THE EXCRETION OF URINE 



urine ceased when the capillary pressure in the glomerulus fell below that 

 exerted by the osmotic pressure of the blood proteins, the critical pressure 

 being from 30 to 40 mm. Hg. They also found that dilution of the blood 

 with saline solution by reducing the osmotic pressure of the proteins in the 

 plasma, was accompanied by an increase in the rate of excretion; excre- 

 tion in such cases being maintained at a blood pressure below the normal 

 critical pressure. If the dilution of the blood was made with saline con- 

 taining gelatin or gum arabic, on the other hand, the diuretic effect was 

 greatly diminished, and any fall in the blood pressure was followed by a 

 suppression of the urine (Knowlton 9 ). These experiments evidently 

 indicate that saline causes diuresis by diluting the plasma proteins and 

 lowering their osmotic pressure, since no diuresis occurs when the os- 

 motic pressure of the blood is maintained by the addition of colloids 

 having an osmotic pressure. The significance of these facts, in connec- 

 tion with the raising of lowered blood pressure after hemorrhage, has 

 already been alluded to (page 140). 



The view that saline diuresis is caused by physical changes alone 

 is confirmed by the experiments of Barcroft and Straub, 10 who showed 

 that the oxygen consumption is often not appreciably raised during the 

 diuresis which occurs after the injection of saline. If the diuresis were 

 due to an actual increase in the work of the kidney, the oxygen con- 

 sumption would have been increased. 



In the frog, the glomerulus and the tubules are supplied with blood 

 by the renal artery, as is the case in the mammal, but the tubules 

 are also supplied with some of the blood coming from the lower ex- 

 tremities and the trunk through a vessel which has no counterpart 

 in the mammal the renal portal vein. The blood, therefore, which 

 is supplied to the tubule is a mixture from the glomerulus and the renal 

 portal system. By ligating the renal vessels it is possible to cut off the 

 blood supply of the glomerulus while leaving the tubules supplied by the 

 renal portal vein. Normally the pressure in the renal portal system is 

 not sufficient to force blood back through the glomerular vessels. Liga- 

 tion of the renal vessels at once results in a suppression of the urine. 



If the glomerular vessels are perfused with Ringer's solution at a 

 pressure equal to that found in the aorta, a considerable flow of fluid 

 may be secured from the ureters, but no fluid is obtained when the renal 

 portal vein is perfused at a pressure equal to that normally present in 

 this vein. Rowntree and Geraghty 11 found that phenolsulphonephthalein 

 added to the fluid perfused through the renal portal vein, did not 

 cause secretion, but when urea was added, fluid containing the dye was 

 obtained from the ureter. Unfortunately the pressure employed in these 

 experiments may have allowed some fluid to be forced backward into the 



