462 



ORGAN SYSTEMS OF MAN 



the liver by tiny tubules, accumulates in one 

 large duct, the bile duct, which eventually 

 joins the pancreatic duct just before the two 

 empty into the duodenum (Fig. 17-2). 

 However, there is a storehouse for the con- 

 tinuously secreted bile, the gall bladder, 

 which is located in a hollow on the under 

 side of the liver. It is a thin-walled sac con- 

 nected to the bile duct by a small duct of its 

 own. Between meals the bile accumulates 

 in this sac and when bile is needed the 

 walls contract, forcing the stored bile into 

 the duodenum in large quantities. 



In so far as digestion is concerned, the 

 only function of the liver is the production 

 of bile. In a sense, bile is an excretory as 

 well as a secretory product because the 

 residue from broken-down red blood cells 

 accumulates in it and is thus eliminated 

 from the body, at least in part. Bile also 

 contains considerable quantities of sodium 

 bicarbonate, which functions in alkalizing 

 the chyme in the duodenum. The organic 

 constituents of bile are bile pigments, bile 

 salts and cholesterol, each of which has 

 specific functions. 



The bile pigments, bilirubin (red) and 

 biliverdin (green), are responsible for the 

 color of bile. While in man bile is straw-col- 

 ored, in other vertebrates it ranges from 

 green to red with all intermediate shades. 

 When mixed with the chyme in the gut, the 

 bile pigments undergo further chemical 

 change, turning to dark brown or black, 

 thus contributing the brown color of stools. 

 The first sign of faulty bile elimination is 

 the gradual loss of this color, and when the 

 bile fails altogether the stools are gray in 

 color. 



Of the numerous constituents making up 

 bile, only the bile salts function in diges- 

 tion. They are responsible for emulsifying 

 the fats in the food so that the fat-splitting 

 enzyme from the pancreas can work more 

 effectively. They also seem to activate this 

 enzyme, because without them the fats are 

 poorly digested and appear in large quanti- 

 ties in the stools. They are conserved by re- 



absorption in the lower end of the small 

 intestine and circulate in the blood back to 

 the liver to be used over again. 



The organic compound cholesterol seems 

 to be important in the bile only because of 

 the trouble it sometimes causes. It does not 

 accumulate if the concentration of bile salts 

 is sufficiently high to keep it in solution. 

 However, if the bile salt level drops, choles- 

 terol will sometimes precipitate out in the 

 gall bladder, forming gaU stones. These are 

 harmless in themselves but if one is forced 

 into the tiny bile duct as the gall bladder 

 contracts in emptying it may occlude the 

 tube and produce trouble. Peristaltic waves 

 in the tube wall attempt to pass the stone 

 along and this causes extreme pain. If the 

 contractions are successful the stone even- 

 tually passes into the intestine where it will 

 do no more harm. If, on the other hand, 

 it remains lodged in the duct, bile fails 

 to reach the intestine and all the symptoms 

 resulting from an absence of bile in the gut 

 ensue. The stools lose their color, fats fail 

 to digest, and the bile pigments, since they 

 are excretory wastes, accumulate in the 

 blood and eventually in the skin, causing it 

 to bronze, or jaundice. Removal of the 

 stones is a simple surgical operation and 

 when accomplished the person is usually 

 restored to health very quickly. 



The pancreas 



This long, flat, light-colored organ lies 

 between the stomach and duodenum, and 

 by means of its pancreatic duct connects 

 with the bile duct. The gland has two func- 

 tions: endocrine, which has already been 

 discussed, and digestive. It produces sev- 

 eral digestive enzymes, all of which are es- 

 sential to complete the digestion started 

 in the mouth and stomach. In addition, the 

 pancreatic juice, like bile, contains large 

 quantities of sodium bicarbonate, which 

 functions in neutralizing the acid chyme 

 from the stomach. Even with this large 

 amount of sodium bicarbonate the chyme 

 remains slightly acid during its trip through 



