PHYSICAL FACTORS OF BLOOD PLASMA OR SERUM 



not diffusible. The following may serve as simple illustrations: 



Take a jar and divide it in two equal parts by an animal membrane, M, 

 figure 124, and place an equal amount of distilled water in the two sides, A 

 and B. Now, since the molecules of water act like those of a gas and are 

 continually moving to and fro, bombarding all the surfaces of their retainer, 

 the molecules of water in A and B will be continually striking all the surfaces 

 of A and B; but since the membrane is permeable to 

 the water molecules, there will be a continual inter- 

 change of molecules between A and B. If now, in one 

 side A we place a solution of sodium chloride, still 

 keeping water in B, the membrane being permeable to 

 the sodium chloride, the first thing we should notice 

 would be an increase in the amount of water in A. For- 



M 



M 



T 



FIG. 124. FIG. 125. 



merly it would have been said that " the salt had attracted the water." Now 

 we should say that the salt had a certain osmotic pressure. The salt, how- 

 ever, being able to pass (dialyse) through the membrane, will do so, and this 

 will continue until the strength of the two salt solutions, and therefore the 

 osmotic pressure on both sides, is equal. 



Osmotic Pressure. If now in A we place a solution of some soluble 

 colloidal substance to which the membrane is impermeable, or else replace 

 the membrane, M, we used in our former experiment by one which is not 

 permeable to the sodium chloride, and arrange our jar as in figure 125, so 

 as to be able to read off any increase of water which may pass into A, we 

 will notice that the amount of liquid in A will continue to increase up to a 

 certain point. Once that point is reached, there will be no further change, 

 since the substance in solution, in A , cannot pass through the membrane as 

 in the previous example. This pressure can be measured and expressed in 

 millimeters of mercury. It is constant for all solutions of this substance 

 that are of the same concentration when measured under like conditions of 

 temperature and pressure, and is called the osmotic pressure of this solution. 



Of the numerous explanations regarding the nature of osmotic pressure 

 which has been more or less satisfactory, a simple one, and one that can 



