THE METABOLISM OF THE CARBOHYDRATES 313 



has been taken will plainly demonstrate that this supposition is 

 correct, although the increased excretion of carbonic acid may 

 not be marked for some time after the meal. As we shall see 

 later, much of the carbohydrate which is not immediately re- 

 quired becomes laid aside in the tissues in a modified form to 

 be afterwards called upon, when required by the organism, to 

 assist in carrying out the processes of metabolism. 



Indeed, very soon after its absorption into the blood it being 

 by this path and not by the lymphatics that carbohydrates are 

 absorbed into the organism the carbohydrate undergoes a change. 

 When absorbed into the blood from the gastro-intestinal tract 

 the carbohydrate is in the form of a reducing sugar, and as such 

 it may be detected in large amount in the portal blood. If, on 

 the other hand, the blood of the systemic circulation be examined, 

 only a very small amount of reducing sugar will be detectable in 

 it, and this is true even after a very liberal carbohydrate diet has 

 been taken. As the result of numerous observations, it has been 

 shown that under normal conditions the systemic blood contains 

 from 0*05-0-2 per cent, of reducing sugar. It has also been 

 found that any increase over this amount of sugar in the blood 

 (which condition is called hyperglycaemia) is at once followed by 

 the appearance of the excess of sugar in the urine ; in other 

 words, a condition of glycosuria is established. With a normal 

 percentage in the blood the kidney allows none of the sugar 

 to pass into the urine, but whenever this percentage is exceeded 

 immediate leakage of the excess through the kidney filter occurs. 



Allowing 5000 c.c. for the amount of blood in a full-grown 

 man, we see that, at most, only about 10 grm. of the absorbed 

 sugar can be accounted for in the blood. Nor can we account 

 for the sugar in a free state in the tissues (e.g. muscles), for if 

 these be examined in a perfectly fresh state only the minutest 

 traces of sugar will be found in them. 



In contrast to the systemic blood, the amount of sugar in the 

 portal blood will be found to show great variation, according to 

 whether absorption of food from the intestine be in progress or 

 not. During fasting, the percentage will be seen to be the same 

 as in the systemic blood, but during absorption to be much 

 higher than this (0-2-0-4 per cent.). 



If we consider what the cause of this difference in their sugar 

 content between the two forms of blood can be, we are at once 



