U© a A bsorption, Metabolism, 

 and the Diet 



ABSORPTION 



The major burden of absorption falls upon 

 the small intestine, and all organic and in- 

 organic food compounds, except water, enter 

 the circulation mainly through the wall of 

 this part of the gut. 



The large absorbing capacity of the small 

 bowel depends upon its very great surface. 

 The intestinal mucosa comes into contact 

 with the chyme throughout a length of 23 

 feet, and throughout this distance the in- 

 testinal wall is richly supplied with blood 

 capillaries and lymph channels. Conse- 

 quently ample opportunity is afforded for the 

 blood and lymph in the intestinal wall to 

 come into equilibrium with the chyme. More- 

 over, the intestinal mucosa does not have a 

 flat surface. In fact, as mentioned earlier, this 

 surface resembles velvet, in that it is studded 

 with numerous hairlike villi, which provide 

 at least a tenfold increase of the absorbing 

 area (Fig. 16-25). 



As may be seen in Figure 18-1, each villus 



is well designed to perform the function of 

 absorption. A network of blood capillaries 

 extends out into the villus, and there is a 

 central lymph vessel at the core. The wall of 

 this lymph vessel, unlike the capillary wall, 

 is perforated by definite flaps, which com- 

 municate with the tissue spaces. The blood 

 flows very rapidly through the capillaries of 

 the villus, but the lymph moves very slowly 

 through its channels. Slowly the lymph filters 

 from the capillaries, seeping through the tis- 

 sue spaces. Then the lymph enters the cen- 

 tral lymph vessel and drains away to the 

 network of lymphatics in the wall of the 

 intestine. (Fig. 18-1). 



The transfer of food compounds from the 

 chyme into the blood and lymph is greatly 

 influenced by the structure of the villi. Essen- 

 tially the chyme is an aqueous solution con- 

 taining the various digested food products, 

 and these compounds are all able to pass 

 into either the blood or the lymph. But a 

 large part of the sugars and amino acids are 

 picked up by the blood stream, because the 



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