the blood stream mainly in the form of glu- 

 cose, small quantities of other monosaccha- 

 rides (fructose and galactose) also get into the 

 portal circulation. On reaching the liver, 

 these other sugars are converted into glucose, 

 which is the kind of sugar best suited to the 

 needs of the tissues generally. 



Regulation of the Blood Glucose Level. 

 During the absorption of a carbohydrate-rich 

 meal, the glucose concentration in the portal 

 blood may rise considerably. But if simul- 

 taneously a sample of blood from the general 

 circulation is analyzed, there is not a corre- 

 sponding rise in the sugar content. The ex- 

 cess sugar has been removed from the portal 

 blood as it flows through the hepatic capil- 

 laries. The liver cells convert the extra glu- 

 cose into glycogen, storing this stable reserve 

 of carbohydrate for the future needs of the 

 body. Thus, the liver of a well-nourished dog 

 or man may contain as much as 20 percent 

 (by dry weight) of glycogen. 



Conversely, after fasting, the portal blood 

 contains very little glucose. Nonetheless, a 

 sample of blood from the general circulation 

 displays a normal sugar content. The differ- 

 ence has been made up in the liver by a 

 hydrolysis of part of the glycogen reserves 

 into glucose. 



The liver, therefore, has the important 

 function of maintaining a constant supply 

 of glucose in the circulation. Normally the 

 blood glucose level remains at about 0.1 per- 

 cent; and this amount is just adequate to sup- 

 ply the needs of the tissues. When the glucose 

 absorbed into the portal blood goes higher, 

 the liver stores glucose as glycogen; but when 

 the blood sugar tends to fall below normal, 

 glucose is mobilized from liver glycogen. 



This blood-sugar control is of great im- 

 portance in the economy of the body. Hyper- 

 glycemia, or excess blood sugar, is wasteful, 

 since the kidneys begin to excrete glucose 

 under this circumstance — and "sugar in the 

 urine" (glycosuria) results. Conversely, a 

 gravely lowered blood sugar (hypoglycemia) 

 is dangerous. Lacking an adequate sugar sup- 

 ply, the tissues cannot function properly. 



Absorption, Metabolism, and the Diet - 341 



The patient sinks into a state of hypogly- 

 cemic shock, which may prove fatal unless 

 glucose is provided quickly — usually by intra- 

 venous injection. 



Some glycogen is stored in the muscles, and 

 a small amount of this muscle glycogen may 

 be drawn upon to maintain the blood sugar. 

 However, muscle glycogen acts mainly as a 

 local fuel deposit, available for muscular 

 work (p. 435). 



The endocrine glands (Chap. 22) cooperate 

 with the liver in maintaining the blood sugar 

 level. Adrenalin (p. 406) stimulates the liver 

 to convert glycogen to glucose; whereas in- 

 sulin (p. 408) prevents a too rapid break- 

 down of glycogen. A diabetic patient, lack- 

 ing insulin, fails to store glycogen in normal 

 fashion; in addition, the tissues of a diabetic 

 partly lose their capacity to utilize glucose. 

 In diabetes, consequently, sugar accumulates 

 in the blood and drains off in the urine. A 

 suitable injection of insulin removes both of 

 these impairments, and the blood sugar 

 comes back to normal. Such injections must 

 be calibrated with the utmost care, however. 

 If the injection is too large, the patient may 

 be thrown into "insulin shock" from the re- 

 sulting hypoglycemia. 



After a meal containing large amounts of 

 sugar, a normal individual may temporarily 

 display hyperglycemia and glycosuria; but 

 this merely indicates that the liver is not able 

 to cope completely with such a sudden flood 

 of glucose, absorbed from the digestive tract. 



Deamination and Urea Formation. The 

 primary function of the amino acids is to 

 enable the tissues to synthesize their essen- 

 tial protein components. An ample protein 

 diet, however, provides the body with an ex- 

 cess of amino acids; and under these circum- 

 stances the extra amino acids are used as fuel. 



Deamination (p. 138) must occur before 

 an amino acid can be fully oxidized; and in 

 mammals, this important catabolic function 

 is delegated very largely to the cells of the 

 liver. During the absorption of a protein- 

 rich meal, a superabundance of amino acids 

 is found in the portal blood. But in the 



