704 ACTION OF SALINE DIURETICS 



pressure was normal. Cushny, however, has confirmed Starling's 

 interpretation by experiments conducted in a different way. He 

 controlled the kidney volume by putting a screw clamp on the 

 renal artery, and found that, when the kidney volume was kept 

 down to the normal, the secretion of urine in spite of the diuretic 

 was unaltered. This experiment is open to the obvious objection 

 that the rate of blood flow through the kidney must have been 

 reduced by clamping the artery. 



Magnus has raised another objection to Starling's interpreta- 

 tion of the mode of action of saline diuretics. For, he maintains 

 that it is not the plethora the alteration in volume of the blood 

 but the hydraemia its alteration in composition which causes 

 diuresis. He performed the following experiment to show that 

 alteration in volume, without alteration in composition, causes no 

 diuresis. He transfused blood from one dog into another until he 

 had increased its blood volume by 84 per cent., and found that 

 no diuresis was caused, although the same changes in kidney 

 volume and vascular pressures were produced as when saline 

 diuretics were injected. On the other hand, if the transfused 

 blood were of abnormal composition, as when obtained from a 

 dog which had received an injection of sodium sulphate, then 

 diuresis resulted. Magnus noticed that much of the fluid of the 

 transfused blood left the vascular system as lymph, and that the 

 fluid remaining in blood-vessels was far more concentrated than 

 normal. It follows that there must have been in this experiment 

 an enormous excess of corpuscles and proteid in the circulating 

 blood which would greatly increase its viscosity, and so might 

 impede the rate of circulation through the kidneys. And that 

 this actually does take place under these conditions has been 

 shown by Sollmann. Cushny tried to avoid this objection by 

 injecting normal serum instead of normal blood, and found that 

 serum injections do cause a slight and slow diuresis. But, of 

 course, it is open to any one to say that serum injections do alter 

 the composition as well as the quantity of the blood. While 

 Magnus's experiment indicates that all forms of plethora do not 

 produce diuresis, it is impossible to deduce from it that saline 

 diuresis is independent of plethora. In fact, it is impossible to 

 say how far in saline diuresis the vascular changes cause the 

 diuresis, or how far they are merely adjuvant, as in the secretion 

 of any other gland. 



