32 Physiology of the Kidney 



absolute standards of reference of the mass of functioning 

 tissue."* 



Except insofar as glucose reabsorption and diodrast ex- 

 cretion may be separately impaired by disease, the relative 

 values of glucose-Tm and diodrast-Tm, as compared with 

 the normal ratio, will afford an indication of the presence and 

 approximate number of aglomerular nephrons in the kidney. 



Glomerular filtration can occur in both normal active 

 nephrons and in the glomeruli of impotent tubules (which 

 by definition are still connected with functional glomeruli) ; 

 again the fraction of the glomerular filtrate furnished by 

 these two adjacent categories cannot be distinguished di- 

 rectly, but it should ultimately be possible to reach a separate 

 evaluation of the number of impotent tubules on the basis 

 of the relative values of the filtration rate and glucose-Tm 

 or diodrast-Tm. 



Inactive tubules will be revealed only indirectly by an 

 increase in diodrast-Tm under experimental or therapeutic 

 conditions which abolish tubular ischemia, just as inactive 

 glomeruli will be revealed only indirectly by an increase in 

 glucose-Tm under conditions whereby glomerular ischemia 

 is abolished. 



'•'Short of complete cessation of filtration, the degree of reduction in filtration rate 

 necessary to cause an individual nephron to drop out of glucose-Tm will be determined by 

 the plasma level of glucose, which fact must be taken into account in the definition of 

 "normal active nephrons". 



Similarly, the degree of reduction in blood flow necessary to cause an individual 

 nephron to drop out of diodrast-Tm will be determined by the plasma level of diodrast, 

 which fact must be taken into account in the definition of the tubular excretory mass. 



The diodrast clearance affords no information on the blood flow to either impotent 

 tubules or scar tissue, but only to intact tubular tissue (the tubular excretory mass); 

 hence it is appropriate for purposes of physiological comparison to relate the diodrast clear- 

 ance to diodrast-Tm. 



A reduction in diodrast clearance may be due to an actual reduction in blood flow, or 

 to a decrease in extraction ratio, due to a reduction for any reason in the excretory ca- 

 pacity of the tubule. If the latter is due to disease the tubule would enter the category of 

 impotent tubules or scar tissue. 



