16 Physiology of the Kidney 



with that of inuKn, but about 5 per cent of this filtered urea 

 escapes from the tubular urine by passive diffusion and con- 

 sequently only half as much plasma is actually cleared of 

 urea as is cleared of inulin. Diodrast (Figure 2d) is cleared 

 from the blood both by filtration and tubular excretion, 

 and hence a much larger volume of plasma is cleared of this 

 substance per minute than could be cleared by filtration 

 alone. 



Thus we may legitimately speak of the inulin "clearance" 

 (=filtration rate) the glucose ^'clearance", the urea "clear- 

 ance" and the diodrast "clearance", as the virtual volume of 

 plasma which is cleared of these particular substances in one 

 minute's time. This virtual volume is quite simply calcu- 

 lated by dividing the quantity of substance excreted in one 

 minute's time (UV) by the quantity contained in each cc. 

 of plasma."' 



I have made this perhaps unnecessarily slow approach to 

 the concept of clearance, because it has been my experience 

 that as soon as the word is mentioned, there may occur one 

 of two adverse reactions: the auditor may infer that a clear- 

 ance is just another empirical renal function test, of which 

 there have been plenty in the history of renal physiology; or, 

 discovering that a "clearance" involves an arithmetical cal- 

 culation based upon concentrations in blood and the rate of 

 excretion, he may shy away into the conclusion that it is in- 

 comprehensible to the non-mathematically minded. Neither 

 inference is correct; the mathematics involved is but simple 

 arithmetic, and the calculation is the only physiological and 

 logical method of evaluating the over-all efficiency of the 



'•'Since most of the substances with which we are concerned do not penetrate the red 

 cells but are carried to the kidneys solely by the plasma, it is desirable, in order to avoid 

 errors due to variable hematocrit, to make all clearance calculations on the basis of the 

 concentration of substance in plasi^na rather than whole blood, and to call such clearance, 

 "plasma clearance." 



