TfiE EXCRETION OF URINE 545 



would act merely as a* filter, the urine having the same concentration 

 of each urinary salt as is present in the blood. 



A comparison of the concentrations of the urinary salts in the urine 

 and the blood shows, however, that the urine is not merely a deprotein- 

 ized blood plasma, so that other factors must be sought to explain the 

 excretion. Since the concentration of the urine requires the expenditure 

 of much more energy than is provided by the known physical factors, 

 it is generally accepted that the renal cell in some manner supplies this 

 energy by its metabolic activity. It is impossible at present even to 

 surmise the nature of the process. Two possibilities may be considered. 

 One is that the urine is a filtrate of the blood which has passed through 

 a portion of the renal epithelium into the tubules as a very dilute fluid, 

 resembling the blood plasma minus its colloidal substances, and that 

 this dilute fluid is concentrated by the reabsorption of fluid and of salts 

 by other cells of the kidney, and again replaced in the blood stream. The 

 other is that the salts and fluid are each actively and individually ex- 

 creted by the kidney. Whichever condition is the true one, the fact 

 remains that the change in the concentration entails the expenditure 

 of a great amount of energy on the part of the renal cells. 



The energy which the kidney must use in the actual work of concen- 

 trating the urine from the fluid of the blood plasma can not be com- 

 puted from a comparison of the concentration of the urinary salts as a 

 whole in both the blood and the urine. Each constituent must be con- 

 sidered apart. We can not, for example, determine the molecular con- 

 centration of the blood plasma and the urine (by measuring A) (page 

 10) and estimate the work which is expended in producing the con- 

 centration from the observed difference. On the basis of such comparisons, 

 however, it is said that the excretion of 100 c.c. of urine requires at the 

 minimum 500 kilogrammeters of work (Cushny 2 ). Even this conserva- 

 tive estimate may be wrong, for it does not take into consideration the 

 possibility that the excretion of water by the kidney requires energy 

 expenditure on the part of the renal cells. 



Theories of Renal Function 



For many years two rival hypotheses have dominated the teaching of 

 the mechanism of renal function. Bowman and Heidenhain postulated 

 that the constituents of the urine are secreted by the vital activity of 

 the epithelium of the capsule and the tubules. The glomerular capsule 

 secretes the water and the easily diffusible salts in a dilute solution, and 

 the uriniferous tubules add to this fluid the various organic and inor- 

 ganic salts to bring the urine to the necessary concentration. This 

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



