QUANTITY AND COMPOSITION OF BLOOD 861 



amount of tartaric acid. The acid mixture is then titrated against a 

 decinormal solution of sodium hydrate until the mixture gives a blue stain 

 when a drop of it is placed on glazed litmus paper. The alkalinity of 

 normal blood as determined in this way amounts on the average to 0-2 grm. 

 NaHO per 100 c.c. of blood. If the blood be laked the alkalinity rises to 

 as much as 04 grm. NaHO per 100 c.c. 



All these questions of reaction depend, however, on the indicator 

 employed. A neutral salt might react alkaline to litmus if the acid radical 

 of the salt were capable of being displaced by the coloured acid radical 

 of the indicator with the production of a blue alkaline salt of litmus. 

 Sodium bicarbonate may be acid or neutral to litmus and alkaline to methyl 

 orange. The absolute alkalinity of any fluid may be expressed by the 

 number of free OH ions which it contains, just as the acidity is a measure 

 of the free H ions. The number of free OH ions in the blood can be deter- 

 mined by an electrical method, and is found to be very small, very little 

 more in fact than that contained in distilled water. The alkalinity of the 

 blood as ordinarily determined by the litmus method gives us, however, 

 more important knowledge than this determination of its absolute alkalinity, 

 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 tissues juices forming the immediate environment 

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

 must be of considerable importance in regulating the normal 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. 125). Of these the most con- 

 venient 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 there- 

 fore 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 distilled water, 



