Renal Function in Man 21 



vitamin C is handled by the human kidney in a manner 

 similar to glucose; the maximal rate of tubular reabsorption 

 averages 2.2 mgm. per minute, which figure we have used in 

 the chart; at an average filtration rate of 125 cc. per minute, 

 vitamin C will first appear in the urine at 1.76 mgm. per 

 cent, and at increasing plasma levels the vitamin C clearance 

 will increase and approach the inulin clearance asymptoti- 

 cally, as in the case of glucose. 



Goudsmit, Power and Bollman'" have recently reported 

 that SOi is handled in a somewhat similar manner by the 

 tubules of dog and man, but too little information is avail- 

 able on the reabsorption of Na, K, Cl, amino acids, uric acid, 

 etc., to say whether these substances will show a maximal rate 

 of tubular reabsorption or not. It would be naive to suppose 

 that all reabsorptive mechanisms in the kidney were as simple 

 as this, and indeed it cannot as yet be said why a maximal 

 rate of reabsorption should limit the reabsorption of glucose 

 or vitamin C. 



Turning now to the process of tubular excretion, the fact 

 that creatinine, phenol red, diodrast and other substances are 

 excreted by the tubules does not exclude the fact that they 

 are also excreted by glomerular filtration. They are excreted 

 through the glomeruli insofar as they are present in the 

 plasma in a filterable form. The creatinine clearance, for ex- 

 ample, has a value of about 40 per cent greater than the in- 

 ulin clearance; since creatinine is completely filterable from 

 the plasma, this means that in addition to 125 cc. of plasma 

 per minute which are cleared of creatinine by filtration, an 

 additional 50 cc. per minute are cleared by tubular excretion. 

 Raising the plasma level of creatinine will not affect the 

 filtration clearance, but it will affect the tubular clearance, 

 for again there is a maximal quantity of creatinine which can 



