GENERAL CHARACTERISTICS OF THE BLOOD 165 



cells can only be retained, if water and nutritive materials are made to 

 move into them, while their waste-products are made to enter the 

 lymph and blood. Osmotic streams are produced, the intensity of 

 which is directly proportional to the difference in the osmotic pressures 

 between the blood and the tissue-fluid. As these pressures are depen- 

 dent in turn upon differences in the concentration of the fluids just 

 mentioned, it is of greatest importance that these differences be 

 retained without allowing them to become too great. This condition, 

 however, is not attained without certain changes, because the body is 

 the frequent recipient of large quantities of water and salts and also 

 loses much water and other material through the secretions and 

 excretions. 



The osmotic pressure of the blood is determined chiefly by its content in crystal- 

 loids, namely the inorganic salts, sugar, urea, and other substances. The pro- 

 teids, however, cannot be said to be without influence. The most convenient 

 method of measuring the osmotic pressure consists in a comparison of the freezing 

 point of the blood with that of water (value = A). A 0.1 molecular solution 

 depresses the freezing point of water 0.186°; and hence, if the depression which a 

 certain solution is capable of producing, amounts to 0.093°, its concentration 

 must be 0.05 molecular. Again, as the former possesses an osmotic pressure of 

 2.24 atmospheres at zero, the latter must exert a pressure of 1.12 atmospheres. 

 For ordinary purposes, therefore, the osmotic pressure of the blood may be meas- 

 ured in a simple manner by determining its freezing point-with an apparatus such as 

 has been described by Hartley.^ The freezing point of mammalian blood is about 

 —0.6° C, which figure implies that its osmotic pressure equals 0.6/1. 85 X 22.4 = 7.3 

 atmospheres. Under normal conditions only slight variations are observed, ^ the 

 values for human blood ranging between -0.52 and -0.60, those for the dog between 

 -0.55 and -0.64, and those for the rabbit between -0.55 and -0.62. 



Curiously enough, some animals are well-protected against the 

 osmotic pressure of the medium in which they live. The bony fish, 

 for example, possess an osmotic pressure of their body fluids which, 

 in the salt water fish, is lower than that of the sea water and, in the 

 fresh water fish, higher than that of the fresh water. This protection 

 which must be ascribed to a peculiar impermeability to water of the 

 lining cells of the gill-plates, enables these animals to migrate. As a 

 typical example of this kind might be mentioned the salmon which 

 enters the fresh water during the spawning period without suffering 

 injury. 



Electrical Conductivity.^ — This factor depends upon the amount of 

 salts present in the blood, because the passage of an electric current 

 necessitates the presence of dissociated ions. Moreover, as the con- 

 centration of the blood varies only within very narrow limits, its con- 

 ductivity must remain almost the same. It should be emphasized, 

 however, that the corpuscles do not permit the current to pass very 

 readily, because they tend to prevent the ions of the salts carrying the 



^ Archives of Diagnosis, 1913. 



2 Hamburger, Osmotischer Druck und Jbnenlehre, Wisbaden, 1902, 456; 

 Hober, Physik. Chemie der Zelle, 1902, 26; also see : Handb. der Physik. Chemie u. 

 Medizin by Koranyi u. Richter i, 1907, 338. 



