Renal Function in Man 1 1 



ratio of urea before and after phlorizin (which drug blocks 

 the reabsorption of glucose) it was concluded that the sugars 

 are not actively reabsorbed by the tubules, as is glucose. On 

 this point we were to prove ourselves in error, for urea, as we 

 now know, is excreted in a variable manner and in it we 

 chose an unreHable standard of reference. But since these 

 carbohydrates consisted of small molecules which might 

 escape from diseased tubules, or from normal tubules at very 

 low urine flows, we sought further in the field of carbohy- 

 drates for a larger molecule, and this line of investigation led 

 us to the use of inulin. 



Inulin is a starch-like polysaccharide composed of 32 

 hexose molecules (mostly fructose) and having a molecular 

 weight of 5200.^^ However, because of the elongate nature 

 of the polysaccharide molecule, its diffusion coefficient is con- 

 siderably less than would be expected from its molecular size, 

 being only twice as great as hemoglobin." It is completely 

 filterable from plasma through collodion membranes, but it 

 is about as large a molecule as one would expect to filter 

 through the glomeruli. It is physiologically inert and rapidly 

 and quantitatively excreted in the urine. 



Our first comparisons of the excretion of xylose and in- 

 ulin revealed that our conclusion that there was no active re- 

 absorption of the sugar was wrong, the concentration ratio 

 of xylose being consistently about 20 to 25 per cent lower 

 than the simultaneous concentration ratio of inulin in the 

 dogfish,^^ sculpin (Clarke, unpublished), dog, sheep, and 

 man.** Unless one postulates the tubular excretion of inulin 

 in the face of much evidence arguing against it, this means 

 that a quarter of the filtered xylose is reabsorbed by the tub- 

 ule. But if an inert sugar, such as xylose or sucrose, can be 

 actively reabsorbed by the tubules, how can one exclude the 



