METABOLISM IN VERTEBRATES 



The monosaccharides glucose, fructose, and galactose are absorbed into the 

 blood in the capillaries of the wall of the small intestine and carried by way 

 of the hepatic portal vein to the capillaries of the liver (Fig. 3.22). Here, 

 under normal conditions, a considerable amount passes into the cells of the 

 liver where it is converted to the complex carbohydrate glycogen for storage. 

 Leaving the liver, the blood still contains more than the normal level of 

 simple sugar. In muscle cells more of it is converted to glycogen, and the 

 blood sugar level soon returns to its normal range of 70 130 mg. per 100 ml. 

 Everywhere in the body as the blood passes through capillaries, glucose en- 

 ters cells and undergoes the metabolic changes discussed previously (p. 35). 

 The level of blood sugar is maintained between meals as a result of the con- 

 version of liver glycogen to glucose which re-enters the blood. Muscle 

 glycogen is utilized during muscle contraction and does not contribute to 

 the blood-sugar level. The human body contains about 200 grams of glycogen, 

 equally distributed between the liver and muscles. Radioactive labeling of 

 glucose reveals that a considerable amount is converted to fat and stored 

 as such. 



Much of the lipid is absorbed into the lymphatic vessels in the wall of the 

 intestine and, in man, is carried to the left subclavian vein by way of the 

 thoracic duct (Fig. 3.23). A sample of blood plasma taken a few hours after 

 a meal rich in fat has been eaten appears milky because of the large number 

 of suspended fat droplets. This fat is promptly stored in the fat depots of the 

 body, such as the subcutaneous and intramuscular connective tissue and the 

 mesenteries of the peritoneal cavity. Stored fat is a readily available source 

 of fuel for the cells. In other words, fat does not go into dead storage; the 

 depots are very labile. It should be noted that stored fat has important in- 

 sulating properties in connection with the conservation of heat and main- 



LIVER 



Dietary 

 lipids 



Absorbed 



lipids 



Structure 



Energy + COg + H2O 



ALL CELLS 

 Fig. 3.23. Transport, storage, and use of lipids. 



77 



