THE SECRETION OF URINE 1275 



neutral crystalloids such as urea or sugar. The question arises whether 

 the chemical changes thereby induced in the renal circulation are 

 sufficient to account for the diuresis. Three factors might be concerned 

 in promoting an increased glomerular transudation. These are : 



(1) A rise of pressure in the glomerular capillaries. 



(2) Acceleration of the blood-flow from the capillaries. 



(3) Diminution of the amount of proteins in the blood-plasma. 

 When a concentrated solution of salt is injected into the circulation 



the osmotic pressure of the plasma is raised and water passes from the 

 tissue-cells into the blood-stream, in consequence of the osmotic 

 differences between the blood and cells so induced. As a result the 

 total volume of the circulating fluid is increased by the addition to it 

 of water derived from the tissues, i.e. a condition of hydraemic plethora 

 is set up just as if a large bulk of normal saline fluid had been injected 

 into the circulation. So long as this hydrsemic plethora continues, so 

 long is there a rise both in arterial and venous pressures and an increase 

 in the velocity of the circulating blood. The kidney placed in an onco- 

 meter shows a great increase in volume. While the plethora lasts 

 there are mechanical conditions at work in the kidneys, i.e. in- 

 creased pressure, increased rate of flow, and diminished concentration 

 of plasma all of which would concur in producing an increased 

 glomerular transudation. With certain salts, such as sodium chloride, \ 

 the diuresis is coterminous with the hydrsemic plethora, but with 

 other members of this class, such as grape sugar, the diuresis outlasts 

 the plethora, so that the continued increased secretion of urine leads 

 to an actual concentration and diminution of the volume of the circu- 

 lating blood, as is shown in Fig. 531. If the kidney be placed in an 

 oncometer, it is found that the dilatation of the kidney outlasts the 

 plethora, and comes to an end only with the cessation of the increased 

 urinary flow. There must be local influences at work (perhaps the i 

 direct effect of the sugar on the blood-vessels) which lead to an 

 active dilatation of the renal vessels, and a consequent rise of 

 pressure and velocity of the blood in the glomeruli. That the 

 vascular change is really responsible for the increased urinary flow 

 is shown by the fact, determined by Cushny, that if the swelling of 

 the kidney be prevented by means of an adjustable clamp on the renal 

 artery, no diuresis is produced ; so long as the kidney is kept at its 

 normal size the flow of urine remains at the same rate as before. 



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 ex- 

 pansion of the kidney occurred without any increase in urinary flow, 

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



