THE l XCR1 TION OF ' RIN1 51 1 



the same concentration as in the blood. In this the renal cells 



would acl merely ;is a filter, the urine having the Bame concentration 

 of each urinary Ball as is presenl in the M I. 



A comparison of the concentrations of the urinary Baits in the mine 

 and the blood shows, however, thai the urine is nol merely a deprotein- 

 ized blood plasma, bo 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 facto 

 it is generally accepted thai the renal cell in sum,' manner supplies this 

 energy by its metabolic activity. Ii is impossible at presenl 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 lias passed through 

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

 resembling the blood plasma minus its eolloidal substances, and that 

 this dilute fluid is concentrated by the reabsorption of fluid and of --alts 

 by other cells of the kidney, and again replaced in the blood stream. Tin -1 

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

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

 remains that the change in the concentration entails the expenditure 

 of a greal amount of energy on the pari 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 <-;m not be cum 

 puted from a comparison of the concentration of the urinary sail 

 whole in both the blood ami the urine. Bach constituent must be con- 

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

 eentration 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 l n " c.c. of urine requires I the 

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

 tive estimate may be wrong, for it does nol take int rideration the 

 possibility thai 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 



• • i 



the mechanism of renal function. Bowman and Eeidenhain postuli 

 thai the constituents of the urine are Becreted by the vital activity 

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



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 lentration. This 



