CHAPTER IX 

 ABSORPTION 



THE term absorption in its restricted physiological use means the 

 process by which the digested foods pass through the walls of the alimentary 

 canal and into the circulation. In a more general sense absorption is the 

 process by which substances pass from one part of the body to another by 

 means other than the blood and lymph vessels. Usually absorption takes 

 place from a free surface, such as the mucosa of the alimentary canal, the 

 surface of the skin, and from the lungs. 



The alimentary canal is lined throughout with a continuous layer of epi- 

 thelial tissue. This layer is only a single cell thick in most of its extent, 

 but nevertheless it effectively separates the food inside the canal from the 

 lymph in the tissue interspaces on the outside of the mucous membrane. 

 These spaces are separated from the blood in the adjacent blood vessels by a 

 second continuous layer, the endothelial walls of the capillaries. The food, 

 therefore, during its absorption from the alimentary canal must pass through 

 two layers of tissue to reach the blood stream. But the submucous lym- 

 phatic spaces and vessels furnish channels which may carry substances into 

 the blood by way of the thoracic duct. The mucous membrane is, therefore, 

 the one strict barrier through which the food must pass in the act of absorption. 



The exact methods by which absorption takes place have long been a 

 subject of controversy and of research. But this problem is of such diffi- 

 culty that it is yet, in the main, unsolved. Known physical and chemical 

 laws are thought to explain the facts of absorption. Some of the phys- 

 ical factors concerned in absorption and elimination have already been 

 considered in a former chapter, osmosis and diffusion, Chapter IV. A third 

 factor, filtration, consists in the passage of a fluid under pressure through a 

 membrane. These factors undoubtedly play an important role in the pas- 

 sage of solutions through the alimentary mucous membrane and the walls of 

 the blood vessels. The part which the physical factors play is probably more 

 pronounced in the absorption of water and crystalloids. The nature of the 

 fluid within the digestive tract, and the movements of the walls of the stomach 

 and intestines by means of which the material to be absorbed is brought 

 into intimate contact with the absorbing membrane, are additional factors 

 which influence absorption. 



But the mechanical and physical factors do not fully explain the observed 

 facts of absorption. It becomes more and more evident that there is an 

 unexplained factor bound up in the characteristics of the living protoplasm 

 of the epithelial cells themselves. When isotonic blood serum is introduced 



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