494 THE HUMAN BODY 



process is carried on by an enzym it is under closer control than 

 the enzym reactions we have studied in connection with digestion, 

 for it does not go on rapidly till all the glycogen is used up, but 

 only so fast as is necessary to make good the loss of sugar from 

 the blood. 



Storage of Glycogen in the Muscles. These organs, as we 

 learned when studying them (Chap. VII), perform their work 

 through the oxidation of sugar, and since they are likely to be 

 called upon for prolonged activity need to have immediately avail- 

 able a supply of their special fuel. Such a supply they have, in 

 the form of glycogen, whi.ch makes up about 1 per cent of the 

 weight of muscle tissue. This glycogen is, of course, derived from 

 the sugar of the blood, so that the muscle-cells must have the same 

 power that liver-cells have of changing sugar to glycogen and 

 glycogen back to sugar. 



The Relation of the Kidney to the Concentration of Sugar in the 

 Blood. As we have seen, the sugar content of the blood remains 

 practically constant all the time at a relatively low concentration, 

 about 0.15 per cent. It is an interesting fact that the kidney, the 

 great excretory organ of the Body, is so constructed that if for any 

 reason the sugar content of the blood rises much above normal, 

 to 0.2 per cent or more, the excess of sugar is withdrawn from the 

 blood by the kidney and appears in the urine. The kidney stands 

 to the sugar of the blood in the relation of a spillway; it allows the 

 concentration to rise just so high, but no higher. This property 

 of the kidney makes such a storage mechanism for sugar as we 

 have described virtually necessary to the Body, since without it 

 the tissues could not be provided with fuel at once continuously 

 and economically. 



Just why the kidney should have this function is not very clear, 

 but a suggestion is found in the observation that the continued 

 presence of excess sugar in the tissue fluids, as in diabetes (p. 496) is 

 inimical to the highest welfare of the tissues. 



The Assimilation Limit. Alimentary Glycosuria. The ability 

 of the liver to convert into glycogen the sugar delivered to it by 

 the portal vein is not without limit. If the absorption from the 

 intestine is so rapid as to raise the sugar content of the portal 

 blood to an abnormally high point, the liver is not able to handle 

 all the sugar; and the excess escapes into the hepatic vein and so 



