THE ABSORPTION AND USE OF FOODS 445 



liver-cells have of changing sugar to glycogen and glycogen back 

 to sugar. Whether the muscles are able to use the sugar of the 

 blood directly or whether they always convert it into glycogen 

 first is not certainly known. 



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 0.2 per cent or more, the excess of sugar is withdrawn from the 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



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 

 into the general circulation. Should this excess be sufficient to 

 raise the sugar percentage of the blood above 0.2 per cent there is 

 excretion of sugar from the kidney, a condition known as gly- 

 cosuria. It is found that the rate of absorption of sugar depends 

 chiefly on how much of it is present at one time in absorbable 

 form in the intestine. Thus if large amounts of single sugar are 

 eaten the essential condition for excessive absorption is likely to 

 be fulfilled. Honey, a sweet containing considerable single sugar, 

 is thus apt to cause glycosuria if too freely eaten. The greatest 

 amount that can be eaten without causing glycosuria marks the 

 assimilation limit. The other carbohydrates, since they require 

 digestion before they are absorbed, are less apt to give rise to too 

 rapid absorption. It is found, however, that there is a great dif- 

 ference in the rate of digestion of the different carbohydrates, and 

 a corresponding difference in the amounts that can be taken with- 



