THE KIDNEY 49 



The introduction of a reliable method for measuring the 

 filtration rate in any animal at any time marks the be- 

 ginning of what may be called the quantitative era in 

 renal physiology. Given this datum, simultaneous meas- 

 urement of the rate at which any other substMice is ex- 

 creted makes it possible to determine whether it is reab- 

 sorbed or excreted by the tubules, and to precisely what 

 extent. 



For measuring the filtration rate a physiologically inert 

 substance is required that (a) is completely filterable 

 through the glomeruli, (b) is neither synthetized nor de- 

 stroyed by the tubules, and (c) undergoes neither addi- 

 tion nor subtraction by the tubules as it passes from the 

 glomeruli to the renal pelvis. The polysaccharide inulin 

 —a soluble, starchlike substance obtained from dahlia 

 roots, the Jerusalem artichoke, chicory, and other vege- 

 table sources— is generally accepted as best fulfilling 

 these specifications. Inulin is nontoxic when properly 

 prepared and adequate doses can be administered to ani- 

 mals and man with ease, while abundant evidence indi- 

 cates that it undergoes neither tubular reabsorption nor 

 excretion in any species. 



InuliQ must be introduced into the plasma by an ap- 

 propriate method (generally by intravenous infusion) 

 and at such a rate that the concentration in the plasma 

 will be maintained at a suitable level throughout the 

 period of observation. Urine is then collected accurately 

 at short intervals, usually ten to fifteen minutes in mam- 

 mals, at much longer intervals in cold-blooded forms. 

 The rate of excretion of inulin (expressed in milligrams 

 per minute) , divided by the quantity in each cubic centi- 

 meter of plasma, gives directly the volimie of plasma 

 filtered per minute— since for each milligram of inulin 

 excreted a corresponding quantity of plasma must be 

 filtered in order to transfer the inulin into the urine. 



This procedure is what is known in renal physiology 

 as the 'clearance method/ since it consists of the deter- 



