THE OSMOTIC FORCE 37 



tion of osmosis is determined by the osmotic pressure difference between the 

 two solutions in contact, but otherwise there is no relationship between 

 osmosis and osmotic pressure. 



The osmotic pressure can be measured by determining the mechanical 

 pressure which must be applied to the solution of high osmotic pressure so 

 that osmosis ceases. The mechanical pressure might be a hydrostatic one 

 (Figure 2-3 (b) i), an elastic restoring force per unit area (Figure 2-3 (b) ii), 

 or some other. 



Water Balance 



In the body (mostly water) the balance among tissues is maintained by a 

 curious assortment of mechanical and osmotic forces, dictated in large part 

 by the physical characteristics of membranes which separate the fluids. All 

 living membranes pass water with ease. It is the solute content which deter- 

 mines the osmotic pressure difference between the two solutions separated 

 by the membrane, and this is determined in part by the membrane itself. 

 Some membranes pass everything— water, salts, molecules — excluding col- 

 loids and larger particles; the large intestine is an example. Membranes in 

 the kidney pass water, salts, and many small molecules readily and rapidly. 

 The membrane which forms the cell wall of the red blood cell passes water 

 and salts, and some small molecules readily. Nerve cell membrane passes 

 water and Cl~ readily, but balks at most molecules (its metabolic rate is 

 low), and lets K + and Na + through only with difficulty. 



Since those species which can pass freely equalize their concentrations on 

 opposite sides, only those which are restricted from passage can give rise to 

 a difference in osmotic pressure. In the erythrocytes, water balance is thus 

 controlled by the difference in soluble protein content between the cellular 

 fluid and the plasma. Since the concentration is slightly greater inside than 

 outside the cell, water runs in. As the cell walls become stretched, the re- 

 storing pressure (the wall is elastic, like a balloon) applies a mechanical 

 pressure on the liquid. An equilibrium is reached at which 



7T, = 7T + P R 



where the 7r's are osmotic pressures inside and outside the cell, and P R is 

 the restoring pressure of the walls of the distended cell. Table 2-1 gives a 

 quantitative illustration of this important point. 



When membranes are ill-formed and cannot discriminate as they should, 

 or when metabolic processes produce impenetrable species such as a protein 

 whose concentration is different from the normal, the osmotic pressure dif- 

 ference, 7r, — 7r , is not the same, and the powerful osmotic force differs from 

 what it should be. The small mechanical compensation mechanisms (such 

 as the restoring force in the erythrocyte wall) become strained, and edema 



