36 



SOME PHYSICAL FORCES EXEMPLIFIED IN MAN 



freezing point, vapor pressure, and boiling point. All of these properties 

 have a value which depends only upon the number of solute particles present 

 in the solution. Thus, pure water has no osmotic pressure; and the greater 

 the concentration (c) of alcohol, for instance, dissolved in water, the greater 

 the osmotic pressure. In fact the osmotic pressure, ir, varies directly as the 

 concentration (number of moles, n, per volume, V): 



7T = —RT = cRT 

 V 



where R is the universal constant and Tthe absolute temperature. Note the 

 analogy with the ideal gas law: 



PV = nRT 



Hence the former could be considered to be an ideal solution law. 



Naturally, the higher the concentration, c, of solute the faster will such a 

 solution diffuse into pure water. However, conversely, the lower the solute 

 concentration the higher is the water concentration, until in the limit, the 

 solution is pure water. Since the laws of diffusion are just the same for water 

 as for any solute, water will diffuse from the solution of higher water con- 

 centration to that of lower water concentration; that is, it will diffuse from 

 the solution of lower salt concentration to the solution of higher salt concen- 

 tration, or, in other words, from the solution of low osmotic pressure to that 

 with high osmotic pressure (see Figure 2-3(a)). It will diffuse from pure 

 water into any solution. The diffusion of water is called osmosis. The direc- 



ts 



(b) 



high TT 

 O 



solute ' 



low TT 



Olo , 



X>o °I°Q 



o 



oU 





ola 



solvent 

 O 



1° 



° .net solven t flow 



O 



>L o 

 ° r\ 



^■membrane 

 _ O 



0:6! o 



hydro- 

 static 

 pressure 



highTT 



(ii) 



lowTI ^^stretched 

 ibrane 



size 



erythrocyte 



membrane 



Figure 2-3. Water Balance, (a) High and low osmotic pressures; (b) osmotic pressure 

 difference balanced by applied mechanical pressure; (i) hydrostatic, (ii) elastic, restor- 

 ing pressures. 



