THE ABSORPTION OF THE REDUCED FOODSTUFFS 1025 



with ease and besides, also the substances in solution. The latter, how- 

 ever, pass with much greater difficulty. For this reason, the osmotic 

 flow of water to the side of the crystalloid is associated with a passage 

 of the molecules of the latter into the water on the other side of the 

 membrane. These counter streams eventually lead to an equalization 

 of the concentration of the fluids on the two sides of the membrane, 

 as well as to an equalization of the osmotic pressure and a cessation of 

 the osmosis. Only diffusion then continues in both directions. 



The osmotic pressure of a solution may be calculated by ascertain- 

 ing the amount of the substance present in it and the degree of the 

 dissociation of its electrolytes. A much simpler method is to deter- 

 mine its freezing point, because the freezing point of water is lowered 

 by substances held in solution, and the degree of lowering is propor- 

 tional to the molecules and ions present in it. A comparison of the 

 osmotic pressures of different solutions may be made by noting their 

 influence upon certain vegetable and animal cells. ^ Thus, if erythro- 

 cytes are brought in contact with the solution to be tested, they either 

 swell, or shrink, or remain normal. Inasmuch as these cells are ordi- 

 narily contained in blood plasma, this medium must be isotonic to 

 them, i.e., it must possess the same osmotic pressure as the red cor- 

 puscles. No osmotic interchanges then take place. It may, therefore, 

 be reasoned that any solution in which they retain their normal size 

 and shape, is isosmotic or isotonic to them as well as to the blood 

 plasma, A hyperosmotic or hypertonic solution is one possessing 

 a greater osmotic pressure, and a hyposmotic or hypotonic solution, one 

 possessing a slighter osmotic pressure than these cells or the blood- 

 serum. In the first instance, these cells will lose water and shrink 

 and in the latter, acquire water and swell up.^ 



Electroljrtes. — The law of osmosis as previously stated, is prac- 

 tically identical with the law of Boyle pertaining to the diffusion of 

 gases. The latter states that the pressure of a gas is proportional to 

 its density, i.e., to the number of the molecules in a given volume of 

 the gas. Like the osmotic pressure, the gaseous pressure remains pro- 

 portional to the absolute temperature and the sum of the partial 

 pressures of the constituents of the mixture. A slight discrepancy be- 

 tween gas pressure and osmotic pressure, however, is produced by 

 the fact that the molecules of many substances, when in solution, are 

 dissociated into two or more parts which are designated as ions. These 

 ions are charged electrically and may be made to arrange themselves 

 in accordance with their potential by passing an electrical current 

 through the solution. Thus, it will be found that sodium chlorid 

 gives rise to Na ions and CI ions, the former being positive and the 

 latter negative. If an electrical current is now passed in a definite 

 direction through this solution, these ions migrate until a perfect 



^ McClendon, Physical Chemistry and Vital Phenomena, 1917, and Bayliss, 

 Principles of Gen. Physiology, 1915. 



2 Overton, Nagel's Handb. der Physiologic, 1907. 



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