The Digestive System - 313 



stead of flowing directly to the duodenum, 

 the bile flows back into the gall bladder, 

 where it is stored until needed (Fig. 16-24). 

 Then later, during the digestion of a meal, 

 the muscular wall of the gall bladder con- 

 tracts, forcing a stream of bile down the 

 bile duct, through which the bile enters the 

 small intestine, together with the pancreatic 

 juice. While the bile is stored in the gall 

 bladder, however, part of its water content is 

 absorbed by the walls of the bladder, so that 

 bile collected from the bladder is more con- 

 centrated compared to the bile that comes 

 directly from the liver. 



The Bile. Although it contains no enzymes, 

 bile plays a very important role in normal 

 digestion. The inorganic components are 

 relatively unimportant, although the sodium 

 bicarbonate of the bile augments the neu- 

 tralizing capacity of the pancreatic juice. 

 Among the main organic components of 

 bile, which include (1) the bile salts, (2) the 

 bile pigments, and (3) cholesterol, only the 

 bile salts are related to digestion. 



The bile of man has a deep yellow-orange 

 color; but the bile of other vertebrates is 

 colored various shades of green, yellow, or- 

 ange, or red, depending upon the propor- 

 tion of bile pigments represented in each 

 case. The main bile pigments are a red com- 

 pound, bilirubin, and a green one, biliverdin. 

 Both of these bile pigments are decomposi- 

 tion products derived from hemoglobin. As 

 the red blood cells of the body gradually dis- 

 integrate and are replaced by new ones, the 

 hemoglobin of the disrupted corpuscles, after 

 its iron (Fe) content has been salvaged, de- 

 composes further, forming the bile pigments. 

 These pigments are then carried in the blood 

 stream to the liver, where they are excreted 

 via the bile into the intestine. In the diges- 

 tive tract, the bile pigments suffer further 

 chemical change whereby the color darkens 

 to brown or black, and the color of the 

 normal stools, or feces, is derived mainly 

 from the bile pigments. Thus when the flow 

 of bile is stopped, as by a gallstone or a 

 catarrhal condition in the bile duct, the con- 



dition is indicated by the occurrence of a 

 whitish, or "clay-colored" stool. Moreover, 

 if the bile pigments accumulate sufficiently 

 in the blood and tissues, the skin of the 

 jDatient shows a yellowish or jaundiced ap- 

 pearance, which confirms the diagnosis that 

 bile pigments are not being excreted prop- 

 erly. 



The importance of the bile salts is related 

 mainly to the digestion and absorption of 

 fats. The bile salts serve mainly as emul- 

 sifying agents, so that the churning move- 

 ments of the intestine are more effective in 

 producing a fine emulsion from the fatty 

 components of the chyme. In such an emulsi- 

 fied state, the fats expose a maximum surface 

 on which the pancreatic lipase can act with 

 high efficiency. Moreover, the bile salts ap- 

 pear to serve as specific activators of steapsin. 



In the absence of bile salts — that is, when 

 the flow of bile is stopped — the absorption, 

 as well as the digestion, of fatty foods is seri- 

 ously impaired, and a considerable propor- 

 tion of fatty foods is lost to the body in the 

 feces. In fact, the bile salts act as very effec- 

 tive detergents, in that they augment the 

 solubility of the fatty acids in the chyme, and 

 this greatly increases the absorbability of the 

 fatty acids. Moreover, the bile salts are care- 

 fully conserved by the body. In fact, the bile 

 salts themselves are absorbed in the lower 

 parts of the small intestine, and are carried 

 back to the liver, which resecretes them into 

 the bile. This regular circulation of the bile 

 salts has been known for many years, al- 

 though its significance was realized only in 

 comparatively recent times. 



The presence of cholesterol in bile is im- 

 portant from a clinical point of view. In cer- 

 tain cases, cholesterol precipitates from the 

 bile, forming the commonest type of gall- 

 stones. The precipitation of such a "stone" 

 occurs in the gall bladder while the bile is 

 undergoing concentration, although stoppage 

 of the bile flow occurs only when a stone 

 finds its way into the bile duct. One factor 

 conducive to such gallstones appears to be a 

 lowered reserve of bile salts, since these com- 



