446 ANIMAL BIOCHEMISTRY 



glycogen and thus is dependent on the gkicose of the extracelkdar 

 fluid. Apparently gkicose is one of the few energy metabolites that 

 can pass from the blood into the brain cells. A sharp drop in blood 

 glucose for any reason causes an immediate response leading to coma 

 and death. Therefore, control of glucose levels in blood is essential 

 in all the higher animals. Several materials participate in this con- 

 trol system. 



In fasting animals blood glucose drops to characteristic levels and 

 changes but little more when the fasting is prolonged unless the re- 

 serves become depleted and death is near. For man this plateau value 

 is in the neighborhood of 70 to 90 mg. of glucose per 100 ml. of blood. 

 The range indicated reflects differences between apparently normal 

 individuals. 



After eating, the glucose content of blood increases somewhat, the 

 exact amoimt depending to some extent on the individual but more 

 so on the nature of the diet. A ration high in glucose, fructose, or 

 sucrose, for example, increases blood glucose until absorption from the 

 food is complete. As long as the level remains below a threshold value 

 (about 160 mg. of glucose per 100 ml. of blood for man), the kidneys 

 prevent loss of more than traces of glucose in the urine. Although 

 glucose is readily dialyzed from the blood, it is almost completely re- 

 absorbed and returned to the blood stream. If the blood levels exceed 

 the characteristic threshold, the kidney is not completely effective and 

 glucose appears in the urine. In some pathological states and with 

 some drugs, the threshold is lowered and glucose is excreted when the 

 blood level is within normal limits. Other abnormalities cause the 

 blood levels to rise above the threshold, and glucose is lost under these 

 conditions also. 



Normal individuals can tolerate the ingestion of 1 g. of glucose per 

 kg. of body weight without the appearance of reducing sugar in the 

 urine. Blood levels rise sharply from the fasting value to 150 mg. per 

 ml. for most persons, and then begin to decline sharply. As the level 

 rises in the blood, the muscle and liver cells assimilate increasing 

 quantities of the sugar and convert it into glycogen. A part of this 

 accelerated utilization is correlated with increased activity in the pan- 

 creas, which secretes the hormone insulin at an additional rate when 

 the glucose content of the blood rises. 



Insulin is one of the major factors in the control of blood glucose. 

 This hormone is a protein whose structure has been much studied and 

 is fairly well established. Normally synthesized by special cells, insulin 

 promotes the conversion of blood glucose into cellular glucose-6-phos- 

 phate. Either insulin increases the penetration of glucose into the 



