THE URINARY SYSTEM. 397 



Diuretics. Substances which, when they enter the blood stream, 

 increase the amount of urine formed by the kidneys are called diuretics. 

 Some act by stimulating the cells of the renal tubules to increased 

 secretory activity ; to this group belong urea and sodium sulphate. 

 The ground for believing that they act upon the tubules is that they 

 increase, not only the flow of urine, but also the amount of oxygen 

 consumed by the kidneys. 



The diuretics of the other group increase the formation of urine, 

 but do not alter the gaseous exchange of the kidney ; they include 

 hypertonic solutions of sodium chloride, potassium nitrate, and other 

 salts. When hypertonic solutions of these salts are injected into the 

 blood, they raise its osmotic pressure and bring about the condition 

 of hydrsemic plethora (p. 240). The volume of the kidney and the 

 capillary pressure in the glomeruli are increased ; and as a result of 

 this rise of pressure, more fluid is filtered through the walls of the 

 glomeruli. It may be readily shown that diuretics such as sodium 

 chloride have no specific action, but that they increase the flow of 

 urine solely by raising the capillary pressure. If the usual action of 

 these diuretics in raising the capillary pressure is prevented, the 

 injection of the diuretic does not increase the amount of urine ; this 

 can be effected by keeping the volume of the kidney constant by means 

 of a screw-clamp placed on the renal artery. 



Some diuretics, such as sodium sulphate, produce their effect partly 

 by direct action on the renal tubules by which they are secreted, and 

 partly by producing hydrsemic plethora. 



Summary. From the foregoing experiments we may conclude, in 

 the first place, that the glomeruli, in all probability, simply filter off 

 from the blood a fluid identical in composition with blood plasma, 

 excluding protein ; the amount of this filtrate normally depends solely 

 upon the capillary pressure in the glomeruli. 



Secondly, the tubules secrete into this fluid urea, uric acid, 

 phosphates, sulphates, and other urinary constituents, with some water, 

 the amount of secretion being dependent on the quantity of these 

 substances reaching the kidney in the blood. This will vary with the 

 metabolic activities of the body, with the result that the kidney 

 removes from the blood the waste products constantly entering it from 

 the tissues, and the composition of the blood is kept practically 

 constant. 



Thirdly, the tubules possess a selective power of absorption, whereby 

 certain substances such as sodium chloride, which are of importance to 

 the body, can be retained when required. In the normal kidney it is 

 probable that all these processes are taking place simultaneously, one 



