THE METABOLISM OF THE CARBOHYDRATES 693 



Strong support has been lent to the idea of the renal threshold by 

 the recent work of Woodyatt and his collaborators, who have shown by 

 continuous intravenous glucose injections that as much as 0.8 gm. of 

 glucose per kilo body weight can be injected during an hour into an 

 animal without any glycosuria, although under such conditions a very 

 distinct increase occurs in the percentage of sugar in the blood. 



To explain the failure of glucose to pass into the urine under normal conditions, 

 it has been supposed by several investigators that the glucose exists in some form of 

 chemical combination in the blood. This compound is believed to behave like a 

 colloid. One of the recent supporters of this view is Allen, who has observed that, 

 when glucose is injected intravenously, it causes diuresis as well as glycosuria; whereas 

 glucose injected subcutaneously or taken by mouth causes neither of these conditions to 

 become developed; indeed it causes for some time after its administration a dimin- 

 ished urinary flow. To explain these differences in behavior between glucose ad- 

 ministered intravenously and that taken in other ways, it is supposed that the glucose 

 molecule in passing through the intervening wall of the capillaries combines with some 

 substance to form a compound which becomes available for incorporation into and 

 utilization by the tissues, glucose in a free state being incapable of utilization. This 

 compound is supposed to be of a colloidal nature, and the substance which combines 

 with glucose to form it is believed to be related to the internal secretion of the pan- 

 creas (see page 710). 



The difficulty in explaining why the glucose of the blood does not constantly leak 

 into the kidney is, however, the only evidence upon which the hypothesis of a blood 

 sugar compound rests. No chemical evidence can be offered in support of such a view. 

 On the contrary, all experimental work indicates that the sugar exists in a free state; 

 but unfortunately even this evidence is not convincing. Thus, it has been found that, 

 when specimens of perfectly fresh blood are placed in a series of dialyzer sacs sus- 

 pended in isotonic saline solutions, each solution containing a slightly different per- 

 centage of glucose, diffusion of glucose, in one or other direction, occurs in all of them 

 save one namely, that in which the percentage of glucose in the fluid outside the 

 dialyzer is exactly equal to the total sugar content of the blood. Such a result can 

 be explained only by assuming that all of the sugar in the blood exists in a freely 

 diffusible state. In its general nature this experiment is analogous to that by which 

 the tension or partial pressure of CO 2 is determined in blood (see page 355). 



It has been shown that glycosuria may sometimes become developed 

 because the kidney fails to hold back the blood sugar even when the 

 percentage is not above the normal so-called renal diabetes. For the 

 diagnosis of this condition a comparison must be made between the 

 sugar concentration of the blood and that of the urine. In order to 

 do this at least two samples of blood must be taken, one of them at the 

 beginning and the other at the end of a period during which urine is being 

 collected. Merely to find that one sample of blood collected before or after 

 or during the period of urine collection contains a normal percentage of 

 sugar, does not necessarily indicate that at some other period while the 

 urine was being produced a temporary hyperglycemia may not have ex- 

 isted. Recent contributions have shown that this condition is of more 

 frequent occurrence than it was previously thought to be. 26 



