THE SECRETION OF URINE 1201 



ACTION OF DIURETICS 



Attempts have been made to solve the problem of renal secretion by 

 studying the action of diuretics, i. e. substances which, injected- into the 

 blood stream or absorbed from the alimentary canal, increase the secretion 

 of urine. These attempts have generally ended in trying to explain the 

 action of diuretics by the theory preferred by the experimenter. A large 

 increase in the urinary flow can be brought about by the intravenous injection 

 of saline diuretics such as sodium sulphate or potassium nitrate, of neutral 

 crystalloids such as urea or sugar. An increased production of urine may 

 be due to augmented glomerular transudation or to increased secretion, or to 

 diminished absorption in the tubules ; and in many cases both mechanisms 

 may be involved. 



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 through 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 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. rise of pressure, greater 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 

 may be coterminous with the hydrsemic plethora, but with other members 

 of this class, such as grape sugar, the diuresis always outlasts the plethora, so 

 that the continued augmentation in the secretion of urine leads to an actual 

 concentration and diminution of the volume of the circulating blood, as is 

 shown in Fig. 548. 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. Since however increased 

 secretion of urine involves dilatation of the tubules, and therefore 

 swelling of the whole kidney, the rise of the oncometer during diuresis 

 is no proof that there is still a greater circulation through the kidney. 

 In fact, however much glomerular change may be concerned in the initial 

 increase in the urinary flow, the terminal increase must be ascribed to the 

 effects of the injected substances on the tubules. As we have already seen. 

 every substance which is not absorbed by the tubules from the glomerular 

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