8 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



The mean of these limiting concentrations is the critical concentration 

 and indicates the strength of each solution that can be added to blood 

 without causing any damage to the corpuscles. This critical concen- 

 tration is not, as might at first sight be imagined, the same as that 

 which is isotonic with the contents of the corpuscles, but distinctly 

 below it. The reason for this becomes apparent if we observe the be- 

 havior of corpuscles suspended in an isotonic solution which is then 

 gradually diluted. As dilution proceeds, the corpuscles distend, until at 

 last their envelopes burst and the hemoglobin is discharged. The lim- 

 iting concentrations of a given salt vary for different corpuscles; thus, 

 the concentration of sodium chloride solution that just causes laking of 

 frog's blood corpuscles is 0.21 per cent, of human blood 0.47 per cent, 

 and of horse blood 0.68 per cent. It is the strength of the corpuscular 

 envelope rather than variations in the osmotic pressure of the contents 

 that is responsible for these differences. 



The above described method of hemolysis, as it is called, can not be 

 used for comparisons of osmotic pressure in cases in which the solution 

 contains substances which alter the permeability of the corpuscular 

 envelop ; for example, it can not be used when urea, or ammonium 

 salts, or certain toxic bodies are present. This very fact is, however, 

 put to a useful purpose in ascertaining whether a given substance does 

 have a damaging influence on the corpuscular envelope by finding whether 

 hemolysis occurs when we suspend the corpuscles in a solution that is 

 isotonic with the corpuscular contents. We can further determine the 

 degree of this toxic influence by estimating by color comparisons 

 (colorimetry) the amount of hemoglobin that has diffused out of the 

 corpuscles. 



Plasmolysis 



An analogous method for determining osmotic pressure is that of 

 plasmolysis, in which the behavior of certain plant cells is observed 

 microscopically while they are in contact with solutions of different 

 strengths. When the surrounding solution is isotonic with the cell 

 contents, the latter fill the cell and extend up to the more or less rigid 

 cell wall (A in Fig. 3) ; but when the solution is hypotonic, the cell 

 contents become detached from the cell wall at one or more places 

 plasmolysis (B and C). The semipermeable membrane in this case is 

 therefore not the cell wall but the layer of protoplasm on the surface 

 of the cell contents. The method can be used only for detecting solu- 

 tions that are hypertonic, for with those that are hypotonic the cells 

 merely become turgid and exert more pressure on the more or less 

 rigid cell wall. Many of the conclusions that have been drawn from 



