512 THE EXCRETION OF URINE 



theory has been termed the vital theory. Ludwig, on the other hand, 

 advanced what is termed the physical theory, which holds that the 

 glomerulus and capsule act simply as a filter, which allows the fluid 

 of the blood plasma to pass through in a very dilute solution and in 

 large amounts. This fluid is concentrated by physicochemical processes 

 on its passage along the urinary tubules to the pelvis of the kidney. 



Both of these theories are inadequate and fail to explain the phenom- 

 ena which research has shown to occur in the kidney, but they have 

 served to develop what Cushny terms a modern theory of urinary 

 excretion. 



The Modern Theory of Urine Formation. This theory accepts the 

 general scheme of filtration and reabsorption of Ludwig, but pays due 

 respect to the fact that the known physical forces are not adequate 

 to explain the reabsorption which must occur in the tubules. It therefore 

 supplements Ludwig 's theory by assuming a vital activity on the part 

 of the epithelium of the tubules in reabsorbing fluids and salts from 

 the dilute filtrate coming from the glomerulus and capsule. A large 

 amount of plasma fluid is filtered through the walls of the glomerular 

 vessels. This fluid has the same concentration of the salts to which the 

 capsule is permeable as does the blood plasma, but it is free of the col- 

 loidal substances normally present in the plasma. The blood leaving the 

 glomerulus is therefore a somewhat concentrated solution of plasma col- 

 loids, and must have returned to it the proper amount of water and 

 salts to make it an optimum fluid for the body cells. This is accomplished 

 by active absorption from the glomerular filtrate. The salts that are of 

 no use to the body are not reabsorbed and therefore appear 'in highly 

 concentrated form in the urine. These salts are termed nonthreshold sub- 

 stances, and since their presence in the plasma is unnecessary, they con- 

 tinue to be excreted as long as they are present in any concentration in 

 the blood. The salts that are necessary for the plasma are termed 

 threshold substances, and are reabsorbed until they are again present in 

 the plasma in optimal strength. For example, urea continues to be ex- 

 creted as long as any is present in the blood, while glucose is completely 

 reabsorbed so long as its concentration remains under a more or less 

 fixed level. 



It is impossible to give a summary of the arguments which have been 

 advanced in support of any of the theories. However, since the modern 

 theory appears to offer a better explanation of the established facts, it 

 may be wise to recount some of the best experimental evidence in support 

 of it. 



First, we must inquire as to the amount of deproteinized blood plasma 

 which the capsule must filter off from the blood in order to furnish the 



