514 THE EXCRETION OF URINE 



debate. Probably both factors are involved, as is shown by the follow- 

 ing observations. If the blood pressure is increased by vasoconstriction 

 in the splanchnic area produced by stimulation of the splanchnic nerves, 

 the flow of blood through the kidney is decreased and the excretion of 

 urine falls. Apparently, secretion can continue only as long as the col- 

 loids of the plasma are not notably increased, for, as the osmotic pressure 

 due to the indiffusible colloids rises, the pressure in the capillaries is no 

 longer able to oppose it. The same point has been beautifully shown by 

 Starling and his pupils, who found that the secretion of urine ceases 

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

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

 indicate that the saline produces its diuresis by diluting the plasma 

 proteins and loAvering their osmotic pressure, since when the osmotic 

 pressure of the blood is maintained by the addition of colloids in which 

 this is present, no diuresis occurs. The significance of these facts, in 

 connection with the raising of lowered blood pressure after hemorrhage, 

 has already been alluded to (page 139). 



This view is confirmed by the experiments of Barcroft and Straub, 10 

 who showed that the oxygen consumption is often not appreciably 

 raised during the diuresis produced by the injection of saline. If the 

 diuresis produced by this means was due to an actual increase in the 

 work of the kidney, the oxygen consumption 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 cu- 

 riously enough are also supplied with some of the blood coming from the 

 lower extremities and the trunk through a vessel which has no counter- 

 part 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- 

 ture 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 



