130 THE BLOOD 



able), their power of dialysis, however, varying considerably. To the second group belong 

 such bodies as have no crystalline form (amorphous). These are generally bodies with a 

 large molecule, which form colloidal suspensions in water, and are only slightly or not at all 

 diffusible. An exception to this second group is hemoglobin, which has a very large mole- 

 cule but is crystalline and is diffusible. The following may serve as simple illustrations: 



Take a jar and divide it in two equal parts by an animal membrane, M, figure 143, 

 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 interchange 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 . Formerly it would have been 

 said that " the salt had attracted the water." Now we should say that the salt had a cer- 

 tain osmotic pressure. The salt, however, being able to pass (dialyze) through the mem- 

 brane, 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 sub- 

 stance 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 1 25, so as to be able to read off any increase of water which may 

 pass into .4, 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 sub- 

 stance 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 have 

 been more or less satisfactory, a simple one, and one that can be easily understood, is as 

 follows: In figure 125 one surface of the membrane is being bombarded by the molecules 

 of a non-diffusible substance mixed with those of a diffusible one (water) ; while the other 

 surface is being bombarded entirely by water molecules. The former condition per- 

 mits less water to diffuse out, since fewer molecules get to the surface of the membrane; 

 while the latter permits all of the molecules which reach it to pass through. 



Osmotic pressure can be estimated in several different ways in addition to the above, 

 viz., the determination of the freezing point of the solution, determination of the boiling 

 point, determination of the electrical conductivity. The results obtained with the various 

 methods agree very closely. The following solutions have the same osmotic pressure: 

 Sodium chloride, 0.64 per cent; potassium nitrate, 1.09 per cent; sugar 5.5 per cent. 



Isotonic Solutions. Solutions that have the same osmotic pressure are called iso- 

 tonic. The term isotonic is a relative one, implying a comparison with some other solu- 

 tion taken as a standard. In physiology it has been customary to take blood-plasma 

 as a standard. A solution of 0.64 per cent sodium chloride is isotonic for the blood -plasma 

 of the frog, and a 0.9 per cent solution for that of man. Further, any solution which is 

 of a lower osmotic pressure than the standard solution is said to be hypoisotonic (hypotonic) 

 in relation to that solution. A solution of a higher osmotic pressure is said to be hyper- 

 isotomc (hypertonic). 



Water passes in the Direction of the Arrows. 

 Hypertonic saline solution (2 per cent) 



I 

 Blood -plasma 



ft 

 Isotonic saline solution (0.64 per cent) 



I 

 Hypotonic saline solution (0.3 per cent) 



