METABOLISM IN VERTEBRATES 



Fig. 3.21. Blood and lymph 

 vessels in the wall of the di- 

 gestive tract; diagrammatic. 

 A, a portion of the entire 

 wall. B, capillaries in a 

 villus, or finger-like projec- 

 tion of the wall of the intes- 

 tine. C, lymphatic in a 

 villus of the intestine. Ar- 

 rows indicate direction of 

 movement of blood and 

 lymph. 



absorbed from the stomach. Only alcohol is freely absorbed from the stomach. 

 The small intestine is the chief organ of absorption. In the upper part of the 

 small intestine the surface of the mucous membrane is greatly increased by 

 folding and, in mammals, by the projection of numerous finger-like villi. In 

 man it has been estimated that there are 5 million villi and 10 square meters 

 of absorptive surface. Amino acids and inorganic salts pass through the 

 epithelial lining directly into the blood stream as a result of diffusion. Glu- 

 cose, on the other hand, must first be converted at the cell surface into glucose 

 phosphate. This phosphorylation under the direction of a specific enzyme 

 requires energy supplied by ATP. The glucose phosphate is rapidly absorbed 

 by the epithelial cells, and glucose soon appears in the blood stream. 



Lipids are often not completely broken down to fatty acids and glycerol. 

 However, under the emulsifying effect of the bile salts, lipid is dispersed into 

 minute droplets, 0.5 micron or less in diameter. These very fine droplets of 

 lipid enter the epithelial cells and pass directly into the lymphatics of the 



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