656 THE MECHANISM OF ABSORPTION 



and to explain as much as we can by the ordinary physical forces. 

 And of these osmosis and diffusion appear to be the most im- 

 portant. But before we can apply these physical forces to 

 absorption we must know the physical conditions which exist in 

 the blood and lymph, into which material is to be absorbed, as 

 compared with those in the gut. That is to say, we must have at 

 least roughly the following data the exact products which have 

 to be absorbed, and the concentrations in which they exist in the 

 gut ; the form in which they reach the blood and lymph, and the 

 concentration in which they already exist in those fluids ; the 

 nature and site of the changes which they undergo between the 

 gut and the blood or lymph. It is almost unnecessary to point 

 out how meagre is our information on these points. Water and 

 salts alone of our food-stuffs reach the blood unaltered, and their 

 absorption need not be further discussed. The others undergo at 

 least a double change. One of these changes certainly takes place 

 in the gut ; the chemical nature of the final products is not 

 beyond dispute, physically they are probably more or less 

 diffusible ; but their concentration in the gut is unknown. The 

 second change takes place during absorption, but its site is 

 uncertain ; it must be somewhere between the gut lumen and the 

 subepithelial capillaries, and we may place it provisionally in the 

 epithelial cells. The nature of these chemical changes is not 

 known for certain ; and with the exception of fat the same is true 

 of the form in which the absorbed substances reach the circulation. 

 But it is clear that we have to look upon the absorption of fats, 

 carbohydrates, and proteids as generally taking place in two 

 stages ; in the first the digested material has to pass into the 

 epithelial cell and undergo change, and in the second this new 

 product has to leave the cell and pass into the circulation. 



Before considering the absorption of the food-stuffs separately, 

 one feature common to all may be referred to. There is no 

 evidence in the case of any one of them that absorption is in any 

 way regulated by the needs of the body ; whatever quantity can 

 be digested will also be absorbed, no matter whether the body 

 wants it or not. And the same is true of the absorption of 

 water ; but in the case of salts there is evidence that the needs of 

 the body may play a part in absorption. This behaviour of the 

 intestine is in strong contrast to that of the kidney, the main 

 excretory organ of the body. 



