196 THE ARMY FOR HOME DEFENCE 



(l, 2, and 3) Increased blood volume, pressure and flow may 

 be considered together. Increase in pressure, etc., produced 

 mechanically without altering the concentration of corpuscles or of 

 colloids, certainly does produce an increased flow of urine, the 

 constituents of which have a concentration approximating to 

 that produced after injection of Ringer's solution. Barcroft and 

 Straube overcame this difficulty very ingeniously. 



They previously removed a quantity of blood equal in volume 

 to the Ringer they were about to inject, thus keeping the blood 

 volume, etc., normal. The diuresis was produced as before, 

 entailing no extra oxygen consumption. 



(4) The addition of blood corpuscles to make up the deficient 

 concentration made no appreciable difference in the flow of urine. 



(5) Knowlton introduced a colloid, gum acacia or gelatine into 

 the perfusion fluid so that the colloidal osmotic pressure of the 

 injected fluid was equal to that of the blood (25-30 mm. of Hg). 

 This prevented the onset of marked diuresis, gelatine being more 

 efficient in this respect than gum acacia. Two causes may be 

 ascribed to the lower efficiency of gum : («) its lower osmotic 

 pressure (5 per cent, gum has an osmotic pressure of about 12 mm. 

 compared with 5 per cent, gelatine whose osmotic pressure is 

 23 mm. of Hg. Bayliss recommends a 7 per cent, solution of 

 gum), {b) Gelatine has a certain water-holding power which is 

 altered by treatment with salts (see Imbibition, Chap. VIII.). 



The conclusion that one would draw from this series of experiments 

 is that the passage of fluid and salts through the kidney by filtration 

 or dialysis is controlled by the concentration of colloids in the blood 

 plasma. 



Sulphate diuresis. Let us consider now a case where oxygen 

 is consumed and, presumably, work done. Sodium sulphate is a 

 diuretic, i.e., causes a free flow of urine. It is less diffusible than 

 sodium chloride, and may be retained by a collodion membrane 

 which will allow the chloride to pass through. Yet in the kidney, 

 the very reverse seems to take place. Sodium chloride acts very 

 much like Ringer's solution, and though after a large injection of 

 NaCl solution the chloride content of the urine rises, it does not 

 materially alter the concentration of the other solutes. On the 

 other hand, the injection of Na2S04 is followed by the secretion of 

 a urine almost entirely an aqueous solution of the sulphate. After 

 the injection of a solution containing equal amounts of chloride 

 and sulphate of sodium, more sulphate than chloride is excreted, 

 and while the chloride elimination falls to normal in about an hour, 

 at the end of three hours the sulphate content of the urine is still 

 above the normal. These results point to the sulphate as having a 



