CHAPTER IX 



ABSORPTION 



ABSORPTION in its restricted use means the process by which the digested 

 foods pass through the walls of the alimentary canal and into the circulation. 

 In its more general meaning 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 the absorption takes place from a free surface, 

 such as the alimentary canal, the skin, and 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, in its absorption, must pass through two layers of tissue to reach 

 the blood stream. But the submucous lymphatic 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 were thought to explain the facts of absorption. Some of the known 

 physical 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 passage 

 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 



361 



