Renal Function in Man 27 



cc. of plasma are required to deliver 700 mgm. of X per min- 

 ute, if each cc. of plasma carries 1 mgm. 



The determination of the extent to which the clearance 

 of any substance approaches completeness (i.e., 100 per cent 

 extraction, as shown at the right of figure 3 ) can only be an- 

 swered at the present time by direct comparison of blood 

 from the renal artery and the renal vein; and these blood 

 samples must be collected under such conditions that renal 

 function is not seriously disturbed. Since anesthesia and 

 handling of the kidney do disturb renal function, this has 

 proved to be a difficult undertaking in man, but we hope ulti- 

 mately to make such determinations under entirely satisfac- 

 tory conditions. However, the plasma diodrast clearance in 

 normal man under basal conditions averages about 740 cc. 

 per minute, which corresponds to slightly under 1300 cc. of 

 whole blood. *^ The basal cardiac output we may take to be 

 4000 cc. per minute. In short, the renal blood flow as given 

 by the diodrast clearance constitutes almost one third of the 

 total cardiac output, and we infer that the actual total renal 

 blood flow cannot be much in excess of this. There are other 

 indirect considerations which need not be detailed but which 

 indicate that the diodrast clearance is essentially a complete 

 one, and until further information is available we may assume 

 that such is the case. 



We have, then, in the plasma diodrast clearance at low 

 plasma levels of diodrast, a measure of the physiologically 

 effective renal plasma flow in cc. per minute.'^ It is only 

 necessary to divide the renal plasma flow by the per cent of 

 plasma in whole blood to obtain the renal blood flow. 



*This measurement is valid only so long as we do nothing to the kidney to change the 

 extraction ratio of diodrast, i.e., so long as we do not impair the process of tubular ex- 

 cretion. Reasons have been given for believing that a change in extraction ratio can be 

 detected by a change in the phenol red/diodrast clearance ratio (47). 



