26 Physiology of the Kidney 



the above evidence, we must attribute to the depression of 

 tubular activity. 



These facts reaffirm my conviction that the inuHn clear- 

 ance is at the level of glomerular filtration in the human kid- 

 ney. It is, however, highly desirable if possible to obtain some 

 substance which has no chemical similarity to the carbo- 

 hydrates, and which will add to or amend the above evidence, 

 and this search will not be abandoned. 



Returning now to the process of tubular excretion (Fig- 

 ure 3) it will be observed that the blood is cleared of sub- 

 stances such as phenol red and diodrast most efficiently at low 

 levels, where the tubules are not loaded to capacity. It is 

 also evident that diodrast is cleared more efficiently than is 

 phenol red. Considered physiologically, there must be an up- 

 per limit to the possible range of clearance values, for the 

 kidneys cannot excrete more of any substance per unit time 

 than is carried to them by the blood in that interval:"* that 

 is, the upper limit of renal clearance values will be that of a 

 substance which is completely cleared from the blood, and 

 this clearance will be identical with the rate at which plasma 

 itself is circulated through the kidneys. ^^ 



This can be made clear by a simple example : if X is a sub- 

 stance which is completely cleared from the renal plasma in 

 a single circulation through the kidneys, and if each cc. of 

 arterial plasma (=systemic venous plasma) contains 1 mgm. 

 of X and if 700 mgm. of X are being concurrently excreted 

 in the urine each minute, it follows that 700 cc. of plasma, 

 no more and no less, must be flowing through the kidneys in 

 each minute's time. It is the simple problem of how many 



''Excluding, of course, storage or synthesis in the renal parenchyma. Synthesis of 

 phenol red, diodrast, etc. is clearly excluded, and storage of these compounds under 

 standard conditions has been ruled out by Smith, Goldring and Chasis (47). 



