Renal Function in Man 9 



absorption or tubular excretion. Once tubular excretion was 

 admitted as a possibility, it was clear that the unravelling of 

 the problems of renal function could not advance beyond the 

 stage of speculation, whether nephrons were investigated in- 

 dividually or en masse, until there was available at least one 

 substance which was known for certain to be neither ex- 

 creted nor reabsorbed by the tubule cells. Once such a sub- 

 stance had been discovered, it could be used as a standard of 

 reference with which to measure the tubular reabsorption 

 of water, and hence to examine the mechanism of excretion 

 of any other substance. Since this question cannot be an- 

 swered in any instance by direct comparison of the absolute 

 quantity of substance filtered with the quantity ultimately 

 excreted, there remains as the only method of examination a 

 comparison of the rates of excretion of various substances 

 relative to their respective plasma concentrations. 



It was with the recognition of these facts that observa- 

 tions on the relative rates of excretion of various substances 

 in the dog were begun in my laboratory in 1929. We started 

 from Marshall's^^ observation that the aglomerular kidney 

 cannot under any circumstances excrete glucose ; presumably 

 the tubule cells have in their evolution never acquired the 

 capacity to excrete this valuable foodstuff. But even assum- 

 ing that the human tubule is also unable to excrete glucose, 

 nevertheless this tubule is obviously able to reabsorb it, since 

 glucose must be present in the glomerular filtrate although it 

 is normally absent from the urine. Consequently we turned 

 to the various non-metabolized carbohydrates, substances 

 which are relatively inert and which are copiously excreted 

 by the kidneys, as representing the type of compound most 

 likely to fulfill the physiological specifications for measuring 

 directly the degree of water reabsorption. Since it is inad- 



