Renal Function in Man 1 7 



kidney in clearing the blood of any particular substance. 

 One point which perhaps adds confusion to the notion of 

 renal clearances in the minds of many is this: the word 

 "clearance" was introduced into renal physiology in 1928 by 

 Moller, Mackintosh and Van Slyke'^ as an empirical means 

 of describing the excretion of urea, which is dependent upon 

 the rate of urine flow. Variations in urine flow arise from 

 variations in the amount of water which is reabsorbed from 

 the glomerular filtrate by the tubules, and in consequence of 

 the variable degree of concentration of the urea in the tubu- 

 lar urine a variable quantity of the filtered urea diffuses back 

 across the tubules. But this is not true of inulin nor of other 

 substances with which we will be concerned here; we may 

 therefore divorce the concept of renal clearance from all as- 

 sociation with urine volume per se and re-define it in the 

 above general terms. Such a generalization of the concept of 

 renal clearance was implicitly effected in 1932 by JoUiffe, 

 Shannon and Smith^^' ^^ in our description of the rate of ex- 

 cretion of various non-metabolized sugars by the dog, and it 

 has proved to be a surprising fluid and useful mode of expres- 

 sion. 



Let us now examine the clearance values of various sub- 

 stances in normal man, these clearances being measured when 

 the substances are present in the plasma at suitably low con- 

 centrations, as shown at the left of Figure 3. These clear- 

 ances vary upwards from 0.0 to slightly above 700 cc. of 

 plasma per minute. The glucose clearance, of course, is zero 

 because at normal plasma levels the filtered glucose is all re- 

 absorbed by the tubules. The same may be said of vitamin 

 C, and the various other substances not shown in the figure, 

 such as amino acids, uric acid, sodium, potassium, chloride 

 and SO4; all of these substances have been demonstrated to be 



