1HO PHYSIOLOGY 



It will be seen that in every case where an increased blood-flow attended 

 with a rise of blood-pressure in the glomerular capillaries, is brought about, 

 the urinary flow is at the same time increased. 



Another factor, altering the ease with which filtration of watery fluid 

 and salts would take place through the glomerular capillaries, would be the 

 composition of the blood-plasma. Any dilution of this plasma must render 

 filtration more easy, while a concentration would make it more difficult. 

 As a matter of fact hydrsemia, and especially hydrsemic plethora caused by 

 injection of normal saline into the circulation, evoke an increased flow of 

 urine. A smaller effect is produced by injection of defibrinated blood, and 

 if the blood has been previously concentrated by depriving the animals of 

 water, there may be little or no increase in flow, in consequence of the high 

 osmotic pressure of the proteins of the plasma injected. 



Experiments on the action of diuretics have a close bearing on the nature 

 of the process occurring in the glomeruli. AJajrge increase in the urinary 

 flow can be brought about by the intravenous injection of saline diuretics 

 such as sodium sulphate or potassium nitrate, or of neutral crystalloids such 

 as urea or sugar. The question arises whether the mechanical 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 hydrsemic 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 oncometer shows a great increase in volume. While 

 the plethora lasts there are mechanical conditions at work in the kidneys, 

 i.e. increased 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 hydraemic plethora, but with other members of this 

 class, such as grape sugar, the diuresis outlasts the plethora, so that the con- 

 tinued increased secretion of urine leads to an actual concentration and 

 diminution of the volume of the circulating blood, as is shown in Fig. 533. 

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



