226 TEXT-BOOK OF PHYSIOLOGY. 



stitutes the force of filtration, and hence the greater the difference, the greater will 

 be the amount of fluid filtered. 



With any given artificially prepared animal membrane the quantities of water 

 and crystalloids in general which pass through the membrane are proportional 

 to the filtration force, and hence the filtrate will have a concentration similar to, 

 if not identical with, that of the original solution. The passage of colloids in solu- 

 tion will be proportional to the permeability of the membrane and an increase in 

 the filtration force. The filtrate, however, will have a lower degree of concentra- 

 tion than the original solution for the reason that as the pressure rises the quantity 

 of water filtered increases in a greater ratio than the quantity of colloid filtered. 



Physiologic Applications. In the animal body the fluids are separated by 

 delicate membranes through which the constituents of the fluids, inorganic and 

 organic, are continually passing. Thus prepared foods in the intestine pass through 

 the intestinal wall into blood- and lymph-vessels; the constituents of the blood 

 pass through the wall of the capillary vessel into the tissue spaces from which they 

 pass, (a) through the walls of various glands to take part in the formation of their 

 secretion; (6) through the sarcolemma into the interior of the muscle fiber; (c) through 

 the limiting surface of, and into the interior of all other tissue cells. The waste 

 products, the result of tissue and food metabolism, pass from the interior of cells 

 through their limiting membranes or surfaces into the tissue spaces; thence through 

 the wall of the capillary vessel into the blood and finally through the wall of the 

 capillary vessel and the epithelium of the lung, the kidney, the liver, etc., to take 

 part in the formation of the excretions. These and other processes are believed 

 to be accomplished by the factors, diffusion, osmosis, and filtration. 



The statements that have been made in foregoing paragraphs in reference to 

 diffusion, osmosis, and filtration have been based on the results of experiments 

 which have been made with non-living membranes, and under conditions purely 

 physical ; and though it is quite true that in the animal body the fluids are separated 

 by membranes more or less permeable to all their constituents, and that all pass 

 through these membranes, it is possible that the facts which have been obtained 

 experimentally are not strictly paralleled in the living body, and hence not strictly 

 applicable to the elucidation of physiologic processes. Nevertheless there are 

 reasons for thinking that a thorough understanding of these factors will eventually 

 throw much light on the intimate nature of the process by which organic as well as 

 inorganic substances in solution pass through animal membranes in the living 

 condition. 



