20 Physiology of the Kidney 



to capacity and some glucose escapes reabsorption and is ex- 

 creted in the urine. A careful analysis of this phenomenon 

 in the dog has been made by Shannon and Fisher/^ and it has 

 been fovind that the tubules are capable of reabsorbing all 

 the glucose from the glomerular filtrate up to a point where 

 this reabsorption reaches a certain maximal rate, in milli- 

 grams of glucose per minute. For brevity we may designate 

 this maximal rate of tubular reabsorption of glucose as glu- 

 cose-Tm. Recent (unpublished) observations on man indi- 

 cate that here precisely the same type of limitation applies; 

 the maximal rate of tubular reabsorption of glucose in man 

 typically ranges around 3 50 mgm. per minute. This figure 

 is easily determined by raising the plasma glucose to a high 

 level, and measuring the simultaneous rate of filtration and 

 of glucose excretion. The curve in Figure 3 represents the 

 glucose clearance in an individual whose filtration rate (in- 

 ulin clearance) is taken to be 125 c.c. per minute, whose max- 

 imal rate of tubular reabsorption of glucose (glucose Tm) is 

 taken to be 320 mgm. per minute, and in whom the plasma 

 level of glucose has been raised from 200 to 800 mgm. per 

 cent. At 25 6 mgm. per cent of glucose in the plasma the 

 rate of filtration of glucose (125 X 256/100 or 320 mgm. 

 of glucose per minute) is just equal to the maximal rate of re- 

 absorption; at plasma levels above this all excess glucose is 

 excreted in the urine, so that the glucose clearance now rises 

 and approaches the inulin clearance asymptotically. Thus 

 the physiological basis of the glucose threshold consists of a 

 constant maximal rate of tubular reabsorption combined 

 with a variable rate of glomerular filtration and of course a 

 variable concentration of glucose in the plasma and there- 

 fore in the glomerular filtrate. 



Ralli, Friedman and Rubin^^ have recently shown that 



