6 PHYSICOCH3MICAL BASIS OF PHYSIOLOGICAL PROCESSES 



Since the osmotic pressure is analogous to the pressure of a gas and 

 is therefore proportional to the molecular concentration (i. e., number 

 of molecules in unit space), it follows that a semipermeable membrane 

 can be used to determine the relative concentration of two solutions of 

 the same substance. When a watery solution of some substance is 

 placed in an osmometer that is surrounded by a similar but more dilute 

 solution, water molecules will diffuse into the osmometer until the pres- 

 sure is equal on the two sides of the semipermeable membrane; that is, 

 the water will pass from the solution having a lower osmotic pressure 

 into the solution having the higher pressure. When two solutions have 

 the same osmotic pressure, they are said to be isotonic; when that of one 

 is greater than that of the other, it is hypertonic; and when less, hypotonic. 



Biological Methods for Measuring Osmotic Pressure 



A practical biological application of these principles can very readily 

 be made if, instead of a rigid semipermeable membrane such as that 

 figured in the diagram, we employ one that is extensible and takes the 

 form of a closed sac ; then as diffusion of water occurs the sac will either 

 distend when it contains a stronger solution than that outside, or shrivel 

 or crenate when the reverse conditions obtain. Many animal and veg- 

 etable protoplasmic membranes are semipermeable, including the en- 

 velope of red blood corpuscles. Thus, if we examine blood corpuscles 

 under the microscope and add to them a saline solution of higher os- 

 motic pressure than blood serum, they will visibly diminish in size and 

 become irregular in shape; whereas if the solution is of lower osmotic 

 pressure, they will distend. If no change occurs, the osmotic pressure of 

 the cell contents must equal that of the saline solution in which the cells 

 are immersed, from which it is clear that we can readily determine the 

 magnitude of the osmotic pressure if we know the strength of the 

 saline solution. 



Instead of measuring the individual cells under the microscope, we can 

 measure the space they occupy in the fluid in which they are suspended. 

 For this purpose a portion of the suspension is placed in a graduated 

 tube of narrow bore, which is rotated in a horizontal position by a cen- 

 trifuge after being closed at one end. The graduation at which the 

 upper edge of the column of cells stands after centrifuging is a measure 

 of the relative amount of cells and fluid in the suspension. Having 

 found this value for cells suspended in an isotonic solution, as for blood 

 corpuscles in blood serum, we may then proceed to ascertain it for the 

 same cells suspended in an unknown solution; if we find that the cells 

 occupy a greater volume, the saline solution must have an osmotic pres- 



