Renal Function in Man 13 



Independently of our own investigations with inulin in 

 a variety of species, Richards and his co-workers, led by the 

 same considerations as ourselves, were examining the excre- 

 tion of this substance in the frog, Necturus and the dog. 

 Hendrix, Westfall and Richards" showed that it is com- 

 pletely filterable through the glomeruli of the frog and Nec- 

 turus. A prehminary report of Richards, Westf all and Bott^^ 

 indicated considerable discrepancy in the way inulin and 

 creatinine are excreted in the dog, but reinvestigation with 

 improved analytical procedures'^ removed these discrepan- 

 cies, so that these investigators as well as Van Slyke, Hiller 

 and Miller'^ are now in agreement with Shannon's'^ '^^ obser- 

 vations that in the dog these two substances have identical 

 concentration ratios within the limits of analytical error. 

 More recently, Richards, Bott and WestfalP° have added to 

 the evidence against tubular excretion of inuHn in the frog, 

 rabbit and dog by perfusing the kidneys of these animals with 

 blood at pressures too low to permit glomerular filtration to 

 occur. Under these conditions hippuran and phenol red, 

 which are indubitably excreted by tubules, accumulate in 

 the tubular urine whereas inulin and creatinine do not. 



Deferring for a moment a further discussion of the ex- 

 cretion of inuhn in man, we will tentatively assume that this 

 polysaccharide is filterable through the glomeruli in the same 

 concentration per unit of water as it is present in the plasma; 

 thereafter it passes down the tubules like a stream of inert 

 marbles, the filtered quantity suffering neither increase nor 

 decrease in consequence of tubular excretion or tubular re- 

 absorption. It follows that the degree of water reabsorption 

 is revealed by the inulin urine/plasma (U/P) ratio, and also 

 that the rate of glomerular filtration is given by the quantity 

 of inulin excreted per minute (UV) divided by the concen- 



