ABSORPTION 153 



by finger-like projections, called villi, which extend into the 

 cavity of the intestine. The villi are supplied with blood ves- 

 sels, arteries, capillaries and veins, and each contains also a 

 lacteal, or lymph vessel. The walls of the villi are very thin, 

 so that only a thin membrane separates the food in the intestinal 

 contents from the capillaries and lacteals. The cells of this 

 membrane are alive, and whereas simple processes of diffusion 

 probably account for a portion of the absorption of food, the 

 living cells of the intestinal wall undoubtedly influence the 

 process by taking up particular substances and passing them, or 

 products made from them, on into the blood in the capillaries 

 or into the lymph in the lacteals.. 



Absorption of Proteins. Proteins are absorbed mainly in the 

 form of amino acids. A portion of these are passed directly into 

 the blood of the capillaries and thus enter the blood stream by 

 which they are carried to the different tissues and cells. Pos- 

 sibly a portion of the amino acids are deaminized, that is, lose 

 their amino groups in their passage through the intestinal wall. 

 Little is known of the further fate of the residue left after 

 splitting off the amino group. Perhaps it is burned by the cells 

 of the body as fuel, or used to build up new substances in the 

 cells. Amino acids have been obtained from the blood or shown 

 to be there by various methods, so that there is no longer doubt 

 that this is the form in which the proteins of the food reach the 

 cells. This subject was long in dispute, for the amount of amino 

 acids in the blood at any one time is so small that only recently 

 has it been possible to prove that they are there at all. 



Carbohydrate Absorption. The monosaccharides glucose, 

 levulose and galactose produced by the digestion of the carbohy- 

 drates are believed to pass into the blood of the capillaries and 

 thence into the blood stream. Glucose is found regularly in the 

 blood to the extent of 0.2-0.4%. The liver takes up monosac- 

 charides and builds them into glycogen, which serves as a reserve 

 store of fuel. Glycogen is stored also in the muscles. 



At times, if excessive amounts of maltose, lactose, or cane 

 sugar are taken, these sugars may get into the blood. The mal- 



