142 ABSOEPTIOK 



Summary of nutrient absorption. — In general, the seat of 

 greatest digestion is the seat of greatest absorption, namely, the 

 small intestine; the large intestine blood vessels taking up only- 

 water and such food as may have escaped absorption in the small 

 intestine. Water, salts, grape sugar, and proteid enter the blood 

 capillaries directly and, except in the case of the proteid, without 

 change. All these nutrients are thus conveyed to a large vein 

 called the portal vein which carries them to the liver. 



Fats enter the jugular vein by way of the lymph capillaries and 

 thoracic duct (the lacteals) in the form of an emulsion. They 

 do not, therefore, enter the liver like the other nutrients. The 

 nutrients which the portal vein carries to the liver are in the form 

 of water, dissolved salts, blood proteid, and grape sugar. 



Action of the liver as an absorbent organ. — The nutri- 

 ents brought to the Uver by the portal vein undergo still 

 further changes before entering that part of the blood 

 circulation which supplies the tissues with food. The 

 most important of these changes is the transformation of 

 grape sugar to a starch or glycogen (sometimes called liver 

 starch). Another change which takes place in the liver 

 is the elimination of poisonous compounds which may 

 be absorbed with the foods, and which, if permitted to 

 circulate to the tissues, would result in disease. 



Glycogen. — The cells of the hver take up the grape 

 sugar brought to it by the portal vein, which itself divides 

 in the liver into many tiny capillaries. This grape sugar 

 the liver changes into a carbohydrate of the same compo- 

 sition as vegetable starch, hence the name liver starch. 

 This substance, like vegetable starch, may be changed 

 into grape sugar again by the action of ptyalin and other 

 diastasic enzymes. Acids also transform it directly 

 into grape sugar. It may be distinguished from starch 

 by its action on iodine solutions to which it gives a wine- 



