1202 



PHYSIOLOGY 



filtrate must act as a diuretic, since it will oppose osmotic resistance to the 

 absorbing powers of the cells. Thus the no-threshold substances, urea, 

 and sodium sulphate, nitrate, and phosphate, will act as diuretics in any con- 

 centration. The threshold substances will act as a diuretic so long as their 

 concentration in the plasma surpasses their normal threshold value. 



180 



100 no uv iso uo 150 



FIG. 548. A comparison of the effects of intravenous injection of 30 grin, glucu.su 

 in concentrated solution on the arterial blood pressure, the concentration of the 

 blood, the kidney volume, and the urinary flow. Abscissa = time in minutes. 



With regard to the specific diuretics, such as caffeine, the question is 

 not quite so clear. In most cases injection of caffeine in the rabbit brings 

 about a dilatation of the kidney and a proportional increase in the secretion 

 of urine. But cases have been recorded in which expansion of the kidney 

 occurred without any increase in urinary flow, and, on the other hand, 

 augmented urinary flow without any increase in the kidney volume or 

 even in the rate of blood flow through the kidney (as determined by Brodie's 

 method). The general rule however is that a greater rate of blood flow is 

 obtained pari passu with the increased urinary flow ; and a consideration 

 of certain peculiarities in the renal circulation must prevent us from laying 



