312 - Multicellular Animals, Especially Man 



serves to neutralize the strongly acid chyme 

 as soon as it leaves the stomach: 



HCI + NaHCO, 



HoO + C0 2 + NaCI 



This function is important because all the 

 digestive enzymes that act upon foods in the 

 small intestine require a neutral or slightly 

 alkaline medium for optimal activity. 



Four en/ymes are present in the pancreatic 

 juice: two proteases, namely, trypsin and 

 chymotrypsin; one pancreatic amylase (also 

 called amylopsin); and one pancreatic lipase 

 (also called steapsin). 



The two pancreatic proteases are endo- 

 peptidases: trypsin and chymotrypsin. These 

 enzymes are particularly effective in the 

 "middle range" of protein digestion: that is, 

 in the hydrolytic splitting of long-chain pep- 

 tides into short-chain fragments. Both, how- 

 ever, can act on full-fledged proteins, such as 

 may have escaped peptic digestion. The spe- 

 cific lytic action of trypsin is upon peptide 

 bonds bordering on the amino acids lysine 

 and arginine (p. 342); whereas the action of 

 chymotrypsin, like that of pepsin, is upon 

 linkages next to tyrosine and phenylalanine. 

 The main net effect of the pancreatic pro- 

 teases, working together, may thus be written: 



Long-chain peptides -f- XHoO 



(alkaline medium) 



trypsin 



chymotrypsin 



Short-chain peptides 



(tripeptides, tetrapeptides, etc.) 



As collected directly from the pancre- 

 atic duct, the pancreatic juice contains an 

 inactive form of trypsin, called trypsinogen. 



The trypsinogen is tranformed into trypsin, 

 however, as soon as the pancreatic juice 

 begins to mix with the intestinal juice, which 

 contains a specific activating enzyme, entero- 

 kinase. 



Pancreatic amylase (amylopsin) plays a 

 very significant role in the digestion of the 

 starchy components of our foods, namely, the 

 various kinds of starch and glycogen (p. 80). 

 Amylopsin is afforded more time than ptya- 

 lin to act upon these compounds, and amy- 

 lopsin works more rapidly than ptyalin. As 

 in the case of ptyalin, the end product of 

 this digestion by amylopsin is maltose. 



Steapsin, or pancreatic lipase, is the only 

 truly active lipase in the whole gastrointes- 

 tinal tract, and in the absence of a proper 

 flow of pancreatic juice, much of the fat in 

 a meal remains undigested and is egested 

 with the feces. The hydrolysis of fat, which 

 may be written: 



1 mol fat + 3 mol H 2 



(alkaline medium) 



steapsin 



1 mol glycerol 



3 mol fatty acid 



is essential if the fatty components of our 

 food are to be properly absorbed and dis- 

 tributed throughout the body. 



The Liver. In vertebrates generally, the 

 liver is a comparatively huge organ, and in 

 man the liver occupies the whole upper part 

 of the abdominal cavity, just below the dia- 

 phragm (Fig. 16-24). In fact the dome-shaped 

 diaphragm, which separates the abdominal 

 and thoracic cavities, arches directly over the 

 liver, coming into contact with a consider- 

 able area on the upper hepatic surface. 



The formation of bile is only one of the 

 many functions of the liver (see p. 313). The 

 bile is formed in all parts of the liver, and 

 is drained into a system of fine ducts that 

 permeate the whole organ. These hepatic 

 ducts lead the bile to a main duct, but in- 



