136 BIOLOGICAL CHEMISTRY 



be the channel of absorption. The great experimental 

 difficulty is that during absorption the difference in con- 

 centration in the blood stream is small. The products 

 absorbed are rapidly removed from the blood stream, hence 

 we can consider that the blood coming to the intestine is 

 always of practically the same composition. During one 

 hour ten litres of blood pass through the blood vessels of the 

 intestine of a dog,* and assuming that as much as 10 grams 

 of sugar are absorbed during one hour, the increase in con- 

 centration of sugar in the blood would be o-i per cent. 



Absorption of Carbohydrates. During absorption of sugar 

 the portal blood shows an increase in glucose above that in 

 the blood passing to the intestine. This excess of glucose is 

 removed by the tissue cells (mainly of the liver) and formed 

 into glycogen.f Glycogen is the form in which carbohydrate 

 is stored in most animals, just as starch is stored in most plants. 

 When carbohydrate is required the glycogen is hydrolysed 

 and the sugar transferred to situations where it is required. 



The experimental data are that following a meal rich in 

 carbohydrate the portal blood going to the liver contains more 

 glucose than the blood coming to the intestine or the hepatic 

 blood coming from the liver. At the same time the amount of 

 glycogen in the liver increases. During the interval, when 

 glucose is not being absorbed the portal blood contains less 

 sugar than the hepatic blood and the amount of glycogen in 

 the liver decreases. 



The mechanism that controls the storage of glycogen can be 

 explained by the law of mass action. There is an equilibrium 

 between glucose and glycogen which tends to keep the con- 

 centration of glucose constant. When glucose is increased in 

 concentration by the arrival of larger quantities of glucose 

 by the portal blood an increased formation of glycogen occurs, 

 but the concentration of glycogen in solution is not increased 

 as it is precipitated in the cells. When the concentration of 

 glucose in the blood is decreased the reverse reaction occurs, 

 and the concentration of glucose in the hepatic blood is 

 increased above that in the portal blood. 



Such a simple chemical explanation requires, however, some 

 modification because the glycogen store in the liver is affected 

 by the splanchnic nerves ; stimulation of the splanchnic 

 nerves causes an increased output of sugar from the liver but 



* R. Burton Opitz., Arch. f. d. ges. Physiol., 1908, vol. 124, p. 469. 

 f Claude Bernard, Lemons de Physiologie experimental appliquce 

 a la Medecine, Paris, 1855. 



