Renal Function in Man 15 



tration in each cc. of plasma (P) . (See Figure 2a) . The fil- 

 tration rate in man, measured in this manner, averages about 

 125 cc. per minute/^ This figure is not surprisingly large 

 when we consider that the total surface area of the glomeru- 

 lar capillary bed, as calculated from Book's^ data on a typical 

 glomerulus, is nearly 1.0 square meter. Out of this 125 cc. 

 of water filtered through the glomerulus, 124 cc. are on the 

 average reabsorbed by the tubules, leaving only 1 cc. to be 

 excreted in the urine. In consequence of the variable reab- 

 sorption of water, inulin (and of course other substances) 

 appears in the urine in variable concentrations, but variations 

 in urine flow or the degree of urine concentration do not 

 normally entail any change in the rate of filtration itself." 



Returning to the overall operation of the kidney, we may 

 say that it is the function of this organ to clear the blood 

 flowing through it of various substances, and that the over- 

 all efficiency of this clearance process in any instance depends 

 upon the specific mechanism of exretion, which in turn will 

 depend upon the properties of the substance under examina- 

 tion. 



Rewording the above discussion, we may say that inulin 

 is cleared from the plasma exclusively by a process of filtra- 

 tion, unmodified by tubular reabsorption or tubular excre- 

 tion. Hence the volume of plasma cleared of inulin in each 

 minute's time must be equal to the rate of filtration itself. 



To extend this idea to other substances, we may say that 

 glucose (Figure 2b) is initially cleared from the plasma by 

 filtration at a rate identical with that of inulin, but because 

 all the glucose is normally actively reabsorbed by the tubules 

 the volume of plasma which is cleared of glucose per minute's 

 time in the overall operation is zero. Urea (Figure 2c) is 

 likewise initially cleared from the plasma at a rate identical 



