972 PHYSIOLOGY 



since on its relative alkalinity to litmus depends to a large extent its 

 power of combining with carbon dioxide and therefore acting as a 

 carrier of this gas from the tissues to the lungs. 



THE OSMOTIC PRESSURE OF THE BLOOD 



Since the blood serves as a circulating medium by means of which 

 the composition of the tissue juices forming the immediate environ- 

 ment of all the cells of the body is maintained constant, its osmotic 

 pressure must be of considerable importance in regulating the nor- 

 mal exchanges of the cells with their surrounding fluid. The osmotic 

 pressure of the blood depends on its molecular concentration and can 

 be determined by any of the methods mentioned earlier (p. 140). 

 Of these the most convenient is the determination of the freezing-point. 

 The depression of freezing-point, A, of mammalian blood is about 0-56 

 and varies between 0-54 and 0-60. The depression of the freezing-point 

 observed in blood is equal to that of a 0-9 per cent, sodium chloride 

 solution, which is therefore taken as isotonic with the blood. Since 

 the corpuscles are in osmotic equilibrium with the plasma, their osmotic 

 pressure must be equal to that of the plasma, and laking the blood 

 does not alter its freezing-point or its osmotic pressure. The blood 

 of the frog has a lower osmotic pressure, the normal saline fluid for 

 the frog's tissues being equivalent to 0-65 per cent, sodium chloride 

 solution. 



THE ELECTRICAL CONDUCTIVITY OF THE BLOOD 



In a solution it is only the dissociated ions which have the power of 

 carrying electric discharges. The conductivity of a solution of pure 

 urea or pure glucose would not differ appreciably from that of dis- 

 tilled water, since neither of these substances is ionised in solution. 

 The conductivity of blood-serum is therefore determined almost 

 entirely by its content in salts. Since this is approximately constant, 

 the conductivity of serum varies within very narrow limits. The con- 

 ductivity of defibrinated blood varies, however, within wide limits, since 

 the outer limiting layer of the corpuscles is impermeable to many of 

 the ions of the salts of the serum. The corpuscles present a resistance 

 to the passage of the charged ions and therefore of the electric current 

 through them, so that the larger the number of corpuscles contained 

 in a given specimen of blood the lower will be the conductivity of the 

 latter. Stewart has made use of this fact as a basis for a method of 

 determining the relative volume of corpuscles and plasma. 



THE ELECTRICAL CONDUCTIVITY OF THE ENTIRE BLOOD AS COMPARED WITH 



THAT OF ITS SERUM. (STEWART.) 



The relative amount of serum can be given by the formula : 



