50 FROM FISH TO PHILOSOPHER 



mination of the volume of plasma which is 'cleared* of 

 a given substance per unit time. If, as in the case of 

 inulin, the substance is 'cleared' from the plasma only 

 by filtration, with no tubular reabsorption or excretion, 

 the rate of plasma clearance (expressed as cc. per min- 

 ute) is equal to the filtration rate. Where a substance 

 is filtered but reabsorbed by the tubules, the rate of 

 clearance vdll be less than the filtration rate; or alterna- 

 tively, where a substance is excreted by the tubules in 

 addition to being filtered, the rate of clearance wdll be 

 greater than the filtration rate. 



Where tubular excretion is present, the rate of clear- 

 ance may be so large that it afiFords a measure of the 

 renal plasma flow— which is the upper limit of any 

 clearance value, since no more of any substance (not 

 synthesized by the kidney) can be excreted into the 

 urine than is brought to the kidneys in the blood plasma 

 per unit time. In principle, the measurement of the renal 

 plasma flow is as simple as the measurement of the filtra- 

 tion rate: given a substance that is readily excreted by 

 the tubules (as well as by the glomeruli), all (or nearly 

 all) of the substance contained in the arterial plasma en- 

 tering the kidney may be removed in a single circulation 

 and concurrently deposited in the urine. If one knows 

 the quantity contained in each cc. of plasma and the 

 concurrent rate of excretion (in mg. per minute), divi- 

 sion of the second figure by the first gives directly the 

 renal plasma flow, and appropriate correction for the 

 hematocrit (or volume of cells in the plasma) gives the 

 renal blood flow. Thus it is possible vdth selected sub- 

 stances to measure the renal blood flow in man and ex- 

 perimental animals wdthout any disturbance other than 

 an intravenous infusion, removal of blood samples from 

 a vein, and the accurate collection of urine by means of 

 a catheter. 



The substance now most widely used for measuring 

 the renal blood flow in man and other animals is 

 p-aminohippmic acid, commonly referred to as PAH. In 



