318 DIGESTION 



nonpolar substances, e.g., fat, while the carboxyl and hydroxyl groups of 

 the molecule have a great affinity for polar solvents, such as water. Bile 

 acids, therefore, have the properties of a detergent, and their mode of 

 action is akin to that of soaps. 



As a result of these chemical properties the bile salts have the ability 

 to increase the water solubility of lipides such as fats and cholesterol, 

 and vitamins A, D, E, and K. The increased water solubility of these 

 otherwise practically water-insoluble materials facilitates their passage 

 through the intestinal wall into other body fluids. The speed of hydrolysis 

 of fats to fatty acids and glycerol in the presence of lipase is increased 

 in the presence of bile salts. 



Once the bile salts have been secreted they are reabsorbed in the in- 

 testines and transported via the bloodstream to the liver, where they 

 are used over again. However, it has been shown with isotopically labeled 

 compounds that the sterol portion of the bile salts can be formed new 

 from administered cholesterol. 



The principal pigments of bile are bilirubin and biliverdin, which are 



M l iV Mfi nP P i i M Mi i V M=CH3— 



11 I 1 A 1 Jl V = CH2=CH— 



HO^^ n'^c'^N^^^N^OH P = -CH.CH.COOH 

 H H H H 



Biliverdin 



products of degradation of heme. The color of feces is mainly due to 



Ml 



V Ml 



P Pn nMMr==iV 



VI I iV Mr nr rn jiMiVir==| v 



H H H2 H H 



Bilirubin 



the products of bacterial reduction of bile pigments, e.g., stercobilin and 

 stercobilinogen. 



The discharge of bile into the duodenum is regulated in part by the 

 nervous system and by secretin. Another substance, cholecystokinin, 

 has been implicated in the contraction of the gall bladder. The presence 

 of the bile salts themselves in the duodenum exerts a powerful stimula- 

 tion on the flow of bile. 



Intestinal secretion 



The intestinal juice is secreted by a large number of glands in the 

 mucosa. This fluid has a reaction of pH 7-8.5. It contains a large 

 number of enzymes; among them are enterokinase (converts trypsinogen 

 to trypsin) , peptidases (hydrolyze peptides to free amino acids), nucleases 



