464 



ORGAN SYSTEMS OF MAN 



secretin can be had by simply injecting 

 an extract from the wall of the duodenum 

 that has come in contact with hydrochloric 

 acid into an animal which has had all of the 

 nerves to the stomach and pancreas cut in 

 order to rule out nervous control. The result 

 is copious secretion of pancreatic juice. 



Bile secretion is stimulated by a some- 

 what similar mechanism. The presence of 

 fats and acid in the duodenum causes the 

 formation of a hormone named cholecysto- 

 kinin, which circulates in the blood and 

 causes the gall bladder to contract and de- 

 liver its contents. 



the small intestine. Therefore, amylopsin 

 is called upon to break these down into 

 maltose, just as ptyalin did, only it does the 

 job more effectively because it has more 

 time. 



Starches + water 



amylopsin 



maltose 



Further degradation of maltose must take 

 place before absorbable glucose is formed. 

 Steapsin does the complete job of digest- 

 ing fats from the complex fat molecule 

 down to fatty acids and glycerol: 



steapsin 



Fats + water » fatty acids + glycerol 



Digestion in the small intestine In this final stage absorption can take place. 



As the chyme spurts through the pyloric ^^^ ^°^^ "«^ ^^^^ ^^^ ^^e intestinal juice 



spincter into the duodenum, it is eventually enzymes is to complete the digestion of pro- 



mixed with bile and pancreatic juice, to- 

 gether with secretions from the intestinal 

 wall called intestinal juice. The intestinal 

 juice contains enzymes that are functional 



teins and carbohydrates. In the case of pep- 

 tids, this is accomplished by erepsin which 

 degrades them to amino acids, ready for 

 absorption. The carbohydrates now in the 

 disaccharide forms of maltose, sucrose, and 



in the final stages of digestion. The chyme 



is rendered less acid by the highly alkaline ^^^*°'^ ^'^ ^^*^^ °" by the corresponding 



nature of both bile and pancreatic juice, enzymes maltase, sucrase, and lactase (col- 



even though it may never reach the neutral 

 stage. Trypsin, as it comes from the pan- 

 creas, is in an inactive form called trypsin- 

 ogen but is converted to the active trypsin 

 the moment it comes in contact with the 

 intestinal juice. The substance responsible 

 for this conversion is enterokinase. Trypsin 

 attacks any complete proteins that have sur- 

 vived the effects of pepsin in the stomach, 

 as well as proteoses and peptones, breaking 

 them down to peptides, thus: 



lectively called invertases) respectively 

 which degrade them to simple sugars that 

 are also readily absorbed by the intestinal 

 wall. 



Small intestine movements 



While the chyme is in the small intes- 

 tine (5-10 hours) it is in constant motion. 

 The purpose of this movement is to mix it 

 thoroughly, and thus insure proper diges- 

 tion and absorption. One of the most impor- 

 tant sources of our knowledge about these 

 and other movements of the alimentary tract 

 has come through x-ray pictures taken dur- 

 These peptides need still further treatment ing various stages in digestion. If a person 

 by enzymes in the intestinal juice before consumes a meal heavily laden with barium 

 they are fit for absorption. (or bismuth), it can be readily followed 



Amylopsin takes up where ptyalin left off through the digestive tract because barium 

 in carbohydrate digestion, because nothing is opaque to x-rays. When observed with 

 happens to these food substances in the a fluoroscope, such a mass appears as black 

 stomach. Ptyalin, as was pointed out earlier, shadows on the screen, and if pictures are 

 does not have a full opportunity to do its taken, the contours of the alimentary canal 

 work so that considerable starch reaches can be made out very easily. This is com- 



Proteoses and peptones -1- H2O 



trypsin 



peptides 



