Renal Function in Man 19 



diodrast, the clearances of which are greater than the inulin 

 clearance, must be excreted by the tubules in addition to be- 

 ing filtered through the glomeruli. 



I have chosen to present a comprehensive picture of renal 

 function in one diagram because it enables us to grasp quickly 

 certain fundamental principles. We can discover important 

 facts about these processes of tubular absorption and ex- 

 cretion by observing what happens to these several clearances 

 when the plasma concentration is changed. Changing the 

 plasma concentration of inulin has no effect on the inulin 

 clearance, since whatever inulin is present in the glomerular 

 filtrate is passed on into the urine regardless of whether it is 

 a large or small amount. The same may be said of urea; the 

 urea clearance is less than the inulin clearance because of dif- 

 fusion out of the tubules rather than because of active re- 

 absorption, and this process of diffusion is determined by the 

 rate of urine flow rather than by the concentration of urea 

 in the tubular urine or the blood. 



But in the case of those substances which are actively re- 

 absorbed or excreted by the tubules, any change in plasma 

 concentration beyond a critical level influences the clearance 

 in a definite, reproducible manner. This dependence on the 

 concentration term arises from the fact that in the operations 

 of reabsorption and excretion the tubule cells can handle only 

 certain limited quantities of any substance per unit time; i.e., 

 these cellular operations are readily loaded to the saturation 

 point. For example, the tubule cells cannot reabsorb an in- 

 finite amount of glucose per unit time; as the plasma con- 

 centration of glucose is increased the concentration of glu- 

 cose in the glomerular filtrate is increased pari passu, and 

 therefore more glucose presented to the tubules per minute, 

 and ultimately there comes a time when the tubules are loaded 



