250 A. I. RINGER AND EMIL J. BAUMAOT 



and if these enter the blood stream pareiiterally, they are quantitatively 

 excreted in the urine. 



The carbohydrates that are absorbable, therefore, are the three mono- 

 saccharides glucose, levulose, galactose and the one disaccharide mal- 

 tose. 



The Sugar of the Blood. That glucose is the most important sugar 

 of the blood we know definitely. Whether levulose and galactose exist 

 in the blood as such is at present not known. From the ease with which 

 these two sugars are converted into glucose when fed to a diabetic indi- 

 vidual, we have every reason to believe that they are converted into glucose 

 either 1 in the process of absorption or soon thereafter. 



Glucose exists in the blood in a state of free solution and not in any 

 chemical union. (Michaelis and Rona (1908).) 



When one examines the blood of an individual for its glucose concen- 

 tration at frequent intervals of time, one finds that under normal conditions 

 it fluctuates within surprisingly narrow limits. In the morning before 

 breakfast, it usually is at its lowest -level, between 0.07 to 0.10 per cent. 

 Between one and one and a half hours after a meal rich in carbohydrates, 

 it rises to a level of 0.10 to 0.15 per cent. After that it gradually comes 

 down, to reach the fasting level about two to three hours after the meal. 

 This cycle of events repeats itself with each meal. 



If a normal individual is allowed to fast for some time, the blood 

 sugar remains about 0.07 per cent and very seldom sinks below that fig- 

 ure. In such cases there is hardly any fluctuation in the blood sugar con- 

 centration from hour to hour. 



There are a number of forces which are operative in keeping the blood 

 sugar concentration at such a constant level, and these are: I. Those 

 that prevent it from rising above normal levels; II. Those that prevent 

 it from falling below normal levels. 



The factors that prevent the blood sugar from rising above normal levels 

 are: 1. Polymerization of glucose into glycogen by the cells of the liver 

 and muscles; 2. Utilization of glucose (oxidation) by the cells of the body 

 for dynamogenetic purposes ; 3. Conversion of glucose into fat. 



The factors that prevent the blood sugar concentration from falling 

 below normal levels are : 1. Mobilization of glycogen from its storehouses 

 liver and muscle and its hydrolysis, which results in glucose formation ; 

 2. Increase in protein metabolism with the result that a large number of 

 ammo acids are converted into glucose. 



The moment sugar enters the intestinal canal its absorption begins. 

 This causes an increase in the glucose concentration of the blood in the por- 

 tal vein. Synchronous with the increase in the portal concentration, there 

 takes place a withdrawal of glucose from the blood by the liver cells and 

 their polymerization of the glucose into glycogen. On the other hand, when 

 absorption of carbohydrates from the intestinal canal has stopped, the 



