340 - Multicellular Animals, Especially Man 



TISSUE SPACE 



LYMPHATIC VESSELS 



Fig. 18-1. Diagrammatic section of two villi, showing 

 the blood and lymphatic vessels. Note that the wall of 

 the central lymph vessel in each villus is imperfect, 

 i.e., perforated by gaps. The interstices among the 

 blood and lymph vessels are occupied by connective 

 tissue, in which the tissue spaces are filled with lymph. 



blood keeps changing rapidly in the capil- 

 laries. Time is not afforded for the blood to 

 become fully saturated with the food prod- 

 ucts as they are absorbed. But the lymph 

 flows so slowly that the concentration of 

 each foods tup soon becomes approximately 

 the same in the lymph as in the chyme. Then 

 there is little or no tendency for the food 

 molecules to enter the lymph, except in very 

 small quantities, as the old lymph is replaced 

 by new. 



The absorption of fatty materials (glycerol 

 and fatty acids) is quite different. The glyc- 

 erol ami fatty acids recombine into fats as 

 they pass through the mucosa of the villus; 

 and this resynthesized fat appears in the 

 form of fine globules in the lymph of the tis- 

 sue spaces — just subjacent to the mucosa. 

 Such lat globules are not able to enter the 

 blood through the capillary walls. Accord- 

 ingly, the fat globules are slowly carried into 

 the central lymph vessel, which conveys them 

 to the lymphatic network. After a fat-rich 

 meal, the lacteals (p. 334), pasing through 

 the mesentery toward the thoracic duct, dis- 



play a conspicuously white, milky hue, indi- 

 cating that the lymph at this time is essen- 

 tially a fatty emulsion. 



Water and inorganic salts — as well as 

 sugars and amino acids — are absorbed pri- 

 marily into the blood stream. But the quan- 

 tity of water leaving the small intestine is 

 more or less counterbalanced by the water 

 entering via the digestive juices. The absorp- 

 tion of salts, to some extent, appears to in- 

 volve active transport processes. In any event, 

 certain salts are absorbed preferentially— to 

 an extent that is difficult to explain in terms 

 of differences in the permeability of the vari- 

 ous ions. Also the absorption of glucose and 

 amino acids is definitely augmented by active 

 transport processes, requiring the expendi- 

 ture of energy by the cells of the intestinal 

 mucosa. 



In summary, the nonfatty food components 

 are absorbed chiefly into the blood of the 

 hepatic portal system, whereas the fatty foods 

 are taken mostly into the lymph. But since 

 the lymph gradually drains into the blood 

 stream via the thoracic duct, the eventual 

 distribution of all the food components is 

 effected by the blood. 



THE LIVER AND ITS SPECIAL METABOLIC 

 FUNCTIONS 



The liver occupies a strategic position in 

 the vertebrate circulation (Fig. 17-16), and 

 on this account the vertebrate liver has as- 

 sumed several important metabolic functions. 

 Probably the liver originated as an ordinary 

 digestive gland, but in modern vertebrates 

 it is an exceptionally large mass of tissue, 

 possessing a wide variety of enzymes. All the 

 blood from the digestive tract passes to the 

 liver capillaries via the hepatic portal sys- 

 tem. Accordingly, the cells of the liver are 

 in position to extract various absorbed prod- 

 ucts from the blood and chemically to alter 

 these compounds before allowing them to 

 pass on into the general circulation, 



Conversion of Other Sugars to Glucose. 

 Although carbohydrates are absorbed into 



