



THE METABOLISM OF CARBOHYDRATES 843 



A similar consumption of glucose occurs in the isolated contracting 

 mammalian heart when fed with Ringer's fluid containing a small trace of 

 glucose. A heart, fed with blood and performing a normal amount of work, 

 may use about 4 mg. sugar per gramme of heart muscle per hour. That the 

 question of utilisation of sugar by the tissues is highly complex is shown by 

 a study of the conditions under which sugar may appear in the urine. We 

 learn thereby to appreciate to some extent the significance of carbohydrates 

 both as sources of energy and as foods for the tissues, though we are still a 

 long way from unravelling all the changes which the sugar must undergo 

 in the cell before it appears once again in the oxidised products, carbon 

 dioxide and water. 



GLYCOSURIA 



Normal urine always contains a small proportion of sugar, about 1 part 

 per 1000, i. e. about the same as the blood itself. For the detection of these 

 small traces of sugar in the urine special methods are necessary. The term 

 glycosuria is not employed unless sugar appears in quantities large enough 

 to give a reaction with Fehling's solution or with the phenylhydrazine test. 

 Such a condition may easily be brought about by the injection of sugar 

 subcutaneously or intravenously. It is then found that any trace of the di- 

 saccharides, cane sugar or lactose, introduced in the circulation, is excreted 

 in the urine. A rather larger quantity of maltose may be injected slowly 

 without appearing in the urine, since the blood serum contains a ferment, 

 maltase, which converts the maltose into glucose. Glucose, fructose, man- 

 nose, or galactose, if introduced slowly into the circulation, are stored up 

 as glycogen in the liver. If however the percentage of sugar in the blood 

 rises above 2 parts per 1000, the sugar (generally glucose) appears in the 

 urine. When this condition of hyperglycaemia (excess of sugar in the blood) 

 is set up, the concentration of the sugar in the urine no longer corresponds 

 to that in the blood. If the blood contains, e. g. 4 parts per 1000, the urine 

 may contain from 2 to 7 per cent, of sugar. Up to a certain point then, 

 blood-sugar is kept back by the kidneys as a necessary food material for the 

 tissues. Any excess above the normal apparently acts as a foreign substance 

 and is excreted by the kidneys in a concentration much greater than that 

 in which it exists in the blood serum. 



(1) ALIMENTARY GLYCOSURIA. A state of hyperglycaemia may be 

 induced by the administration of abnormally large quantities of glucose 

 by the mouth. The amount has to exceed in a healthy individual 

 100 grm. in order that it shall appear in the urine. In certain individuals 

 the power of assimilating glucose may be deficient so that an alimentary 

 glycosuria may be caused by any over-indulgence in carbohydrate food. 

 In the healthy person it is hardly possible to produce glycosuria by the 

 administration of starchy foods, since the liver can store up the excess of 

 glucose as fast as it is produced from the starch by digestion and absorbed 

 into the blood stream. 



(2) DIABETIC PUNCTURE. It was shown by Claude Bernard that 

 puncture of the floor of the fourth ventricle in rabbits is often followed 



