DIGESTION 439 



brane. The bile salt complexes dissociate as soon as they are ab- 

 sorbed, and tissue lipases resynthesize glycerides from the absorbed 

 glycerol and the free fatty acids released from the bile salt-fatty acid 

 complexes. The bile salts eventually return to the liver, where they 

 again become normal bile constituents. 



Although the fatty acid-bile salt complex explanation for fat ab- 

 sorption is generally accepted, some unhydrolyzed fat is definitely 

 absorbed in highly emulsified form, with globules so small that they 

 can pass through the intestinal mucosa. In fact, much if not most of 

 the fat absorbed during digestion is absorbed as finely emulsified un- 

 hydrolyzed glycerides. As suggested in a previous paragraph, a large 

 proportion of the absorbed or resynthesized fat is shunted around the 

 liver by being carried to the tissue via the lymphatic system. This 

 fat-rich lymph is carried by the thoracic duct, where it is poured into 

 the blood stream at the jiuiction of the jugular and subclavian veins. 

 This creamy lymph is called chyle. 



Carbohydrates are absorbed by the capillaries of the villi in the 

 form of glucose, fructose, and galactose, and these simple sugars are 

 carried by the blood stream to the liver, where they are deposited as 

 glycogen (animal starch), or they are taken to the tissues, where they 

 are utilized or deposited as glycogen. Some disaccharide absorption 

 may take place, but it is considered of minor importance. Although 

 glucose, fructose, and galactose may be converted in the liver and 

 muscles to glycogen, it should be noted that tissue glycogen undergoes 

 hydrolysis only to glucose. 



Lactose, because of its relatively low solubility, is not utilized so 

 efficiently as other sugars. As a result an appreciable amount of lactose 

 finds its way to the lower intestine, where it often plays an important 

 role in increasing the lactic acid-forming microorganisms of the intes- 

 tinal flora. The metabolic products of these organisms tend to dis- 

 courage the growth of putrefactive species in the intestine. 



It is now believed that proteins are primarily absorbed in the form 

 of amino acids. While it is possible that some peptides or even poly- 

 peptides may be absorbed, peptidases in the cells of the intestinal 

 mucosa effectively convert these intermediates into amino acids. Since 

 the origin of blood peptides is unknown, perhaps some of these mate- 

 rials occur by escaping hydrolysis. Such a possibility is reinforced by 

 the connection of ingested proteins with food allergies. Either pro- 

 teins themselves or large fragments are thought to be absorbed on 

 occasion, leading to allergic responses in sensitive individuals. 



During processing in the small intestine, peristaltic waves move the 

 contents along until the large intestine is reached. The digestive 



