THE METABOLISM OF CARBOHYDRATES 



799 



THE UTILISATION OF SUGAR IN THE BODY 



Arterial blood is always found to contain between 0-12 and 0'15 per 

 cent, of sugar in the form of glucose. The same amount is found whether 

 the blood be taken from an animal after a heavy carbohydrate meal or 

 from one in a condition of complete starvation. The constancy of the 

 sugar content of the blood suggests that this substance is a necessary con- 

 stituent of the circulating fluid, necessary, that is to say, for the nutrition of 

 the tissues. That it is being used up in all the processes of the body is 

 shown by the immediate alteration in the respiratory quotient which occurs 

 when the food is changed from a mixed diet to one consisting mainly of 

 carbohydrate. An important factor in the maintenance of a constant sugar 

 content in the blood is the reconversion of the stored-up glycogen of the 

 liver into sugar. The glycogen is not, however, the sole source of the sugar, 

 since in complete starvation the sugar content of the blood remains constant 

 even after the last traces of glycogen have disappeared from the liver. If the 

 liver be cut out of the body or removed from the circulation, during the 

 few hours that the animal survives there is a steady diminution in the blood- 

 sugar, pointing to the liver being the chief, if not the sole, source of the blood 

 sugar. In some animals, e.g. the carnivora, it would seem that the liver 

 can continue to supply sugar to the blood on a diet which includes only 

 proteins and fats, and we have already seen that in such animals glycogen 

 itself can be stored up at the expense of protein. It is doubtful whether a 

 perfectly normal existence is possible in man in the total absence of carbo- 

 hydrates from the food, though there is no doubt that in the northern nations, 

 e.g. the Eskimos, the amount of carbohydrate consumed is very small in 

 comparison with the fats and proteins. During muscular exercise the 

 increased output of energy is associated with a corresponding increase in the 

 absorption of oxygen and in the output of carbon dioxide, pointing to a 

 consumption of carbohydrate and fat in the contracting muscles. We might 

 therefore assume that sugar is being normally released by the liver into the 

 blood-stream so as to maintain the proportion of this substance in the blood 

 at a certain level, and that the sugar is as constantly being taken up and oxi- 

 dised in the muscles, where it serves as a source of energy. According to 

 Chauveau and Kaufmann the venous blood flowing from a contracting 

 muscle contains less sugar than the arterial blood flowing to the muscle. A 



