OSMOTIC PRESSURE OF CELL CONTENTS 267 



slightly exceed this concentration cause shrinkage of the corpuscles, 

 while solutions which are less concentrated than blood-serum cause 

 swelling of the corpuscles. 



Assuming the corpuscles in normal serum to be withstanding no 

 pressure or, at the most, a very slight one, it is of some interest to 

 calculate from the degree of hypotonicity the pressure which is required 

 to rupture the corpuscles so as to discharge hemoglobin into the solu- 

 tion. The solutions which are just sufficiently concentrated to prevent 

 rupture are, as we have seen, isotonic with a one-tenth molecular 

 sodium chloride solution or, which comes in terms of osmotic pressure 

 to the same thing, a one-fifth molecular solution of sugar. When 

 neither swollen nor shrunken these cells are isotonic with a one third 

 molecular solution of sugar. The degree of hypotonicity required to 

 rupture the cells therefore, corresponds to the pressure exerted by a 

 3 i = A molecular solution of sugar, i. e., to a pressure of about 

 three atmospheres. 



From the data which we have cited the surface of a red blood-cor- 

 puscle would appear to afford an example of a strictly semipermeable 

 membrane. Here again, however, semipermeability is relative and 

 not absolute. Not only water can enter the cells with ease but also 

 other substances with varying difficulty. An ingenious method of 

 illustrating this fact is that which has been devised by Hedin. 



A measured amount of the substance for which the permeability of 

 the corpuscles is to be tested is dissolved in defibrinated blood, i. e., 

 in a mixture of serum and corpuscles. The serum of this blood will 

 be found to freeze at a lower temperature than untreated serum, 

 because a certain proportion of an additional diffusible substance is 

 contained in it. The depression of the freezing-point of this serum 

 may be designated "a." Now to an equal volume of serum which 

 does not contain any corpuscles an equal amount of the same substance 

 is added. This serum will also freeze at a lower temperature than 

 normal serum, and the depression of the freezing-point which it exhibits 

 may be designated "b." Now, it is evident that if the substance 

 which was added to the defibrinated blood penetrated the corpuscles 

 and dissolved in them to the same extent as in an equal volume of 

 serum, the concentrations of the substance in the two samples of 

 serum would be equal to one another, and we would have a = b. If the 

 blood corpuscles in the defibrinated blood took up less of the dissolved 

 substance than an equal volume of serum, then the substance would 

 be present in greater concentration in the first sample of serum than in 

 the second, and we would have a > b or jj > 1. If, on the other hand, 

 the blood-corpuscles took up more of the dissolved substance than an 

 equal volume of serum then we would have a < b or ^ < 1 . 



The results of this method show that the salts of the alkalies and 

 alkaline earths and the amino-acids and sugars penetrate the corpuscles 

 with great difficulty. Ammonium Salts and Urea, however, pass into the 



