CHAPTER IX 



TRANSFERENCE OF FOOD MATERIALS: 

 ABSORPTION 



THE first stage in the transference of food materials is the 

 conversion of non-diffusible into diffusible substances. 

 This change takes place by hydrolysis in the cells of the 

 leaf or of storage depots in plants and in the alimentary canal 

 of animals. 



The second stage is the building up of these diffusible 

 substances into the non-diffusible constituents of other cells. 

 In plants this is brought about by the sap conveying the 

 materials to the place where they are to be used, but in 

 multicellular animals we have to deal with the absorption of 

 materials from the intestine and their transport in the blood. 



ABSORPTION FROM THE INTESTINE 



The various products formed during digestion are absorbed 

 from the intestine into the blood and lymph streams. The 

 conditions are such as to favour diffusion and osmosis. The 

 diffusible substances are produced in the intestine, thus as 

 digestion proceeds their concentration has a tendency to rise 

 inside the intestine. The blood stream flows continuously 

 in the wall of the intestine, hence any absorbed material is 

 carried away and the concentration of the diffusible substance 

 is kept low. Nevertheless, the absorption is not merely a 

 physical process, as Waymouth Reid has shown that removal 

 of the intestinal epithelium decreases the rate of absorption* 

 and does not increase it as one would expect if the process were 

 purely physical and dependent on the distance between the 

 intestinal contents and the blood stream. Brodie and others 

 have shown, moreover, that there is an increased oxygen 

 consumption during absorption f ; this again indicates that 

 the cells perform work during the process of absorption. 



* E. W. Reid, Phil. Trans., 1900, B. vol. 192, p. 211. 

 f T. G. Brodie and H. Vogt and T. G. Brodie, W. C. Cullis and W. D, 

 Halliburton, Journ. Physiol., 1910, vol. 40, pp. 135 and 173. 



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