ANIMAL METABOLISM 



325 



it can begin to undergo "biological burning," just as a match heats a 

 piece of paper to its kindling point so that it will burn. 



The other reactions shown in Fig. 13-1 are reversible, equilibrium reac- 

 tions (note double arrows) . Such reactions go either in one direction or 

 the other, depending on the relative amounts of the various reacting 

 substances present. Thus after a meal, when a large amount of sugar 

 comes into the blood stream, a considerable part is converted into glycogen, 

 but when the sugar phosphates are consumed during exercise, the glycogen 

 is broken down again. 



Glycogen may also be formed from a variety of other substances which 

 are involved in the further metabolism of carbohydrates (see below). 

 Consequently, the amount of glycogen present in the body at a given 

 time reflects a balance between the intake of all glycogen-forming food 

 materials and the metabolic consumption of carbohydrate as energy 

 sources. 



The amount of glycogen which can be stored, however, is limited. In 

 a normal human adult, the top level of glycogen is seldom over 6 per 

 cent in the liver and 0.7 per cent in the muscles. These percentages cor- 

 respond to a total quantity of about 110 g. in the liver and 250 g. in 

 the muscles. Consumption of additional amounts of food above those 

 needed to maintain this amount of glycogen in the body leads, as is 

 well-known, to the formation of fat. . ' 



Blood Sugar Level. There is also a close interrelationship between gly- 

 cogen, blood sugar, and the action of several hormones. The only sugar 

 which is present in appreciable amounts in the general blood circulation 

 is D-glucose, which for this reason is often called blood sugar. The 

 blood glucose supply is furnished partially by direct absorption from the 

 intestine, but mainly by hydrolysis of D-glucose-6-phosphate coming from 

 glycogen: ^ 



(phosphatase) 



H2O + D-glucose-6-phosphate > D-glucose + H3PO4 



The hormone adrenalin acts to increase the amount of glucose in the 

 blood-stream (p. 289) . Adrenalin is secreted by the adrenal gland in 

 response to intense emotions such as rage or fear. It is usually assumed 

 that this secretion represents a physiological preparation for intense 

 muscular activity to cope with the situation which aroused the emo- 

 tion. 



Two other hormones, insulin from the islets of Langerhans in the 

 pancreas and the diabetogenic hormone from the anterior pituitary gland, 

 also affect the blood sugar level. It is claimed that the latter hormone is 



^ Blood sugar directly absorbed from the intestine may also be formed by hydrolysis 

 of a glucose phosphate, since phosphorylation probably occurs during absorption. 



