OF VITAL PHENOMENA 



47 



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Blood 



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Fig. i8. Curves showing change of PH with change of C0 2 pressure. 

 The C0 2 tension is in percentage of atmosphere (from JBC). 



be replaced by a side outlet with stopcock and the fluid forced 

 into the electrode by immersion in the beaker. 



By the buffer value or reserve alkalinity of the blood, we mean 

 its resistance to change of reaction. Since the buffer value is 

 always expressed in arbitrary units, varying with the method 

 of determination, those values only are comparable that are ob- 

 tained by the same method. The buffer value of serum, although 

 not the same as that of blood, is approximately proportional to 

 it, and hence may be used in place of it (see Fig. 18). Since 

 C0 2 is the principal reagent used by the organism in maintaining 

 the reaction of the blood, it is of interest to know the change 

 in reaction of the blood with change in C0 2 pressure. This is 

 shown in the curve in Fig. 18. It may be noticed that within the 

 normal physiological limits of C0 2 tension the change in reaction 

 is very small (the normal curve passing through PH = 7.5 and 

 C0 2 = 5.2 per cent). It is therefore necessary to take two very 

 different C0 2 pressures in order to obtain a reliable buffer value. 

 But since the PH of blood in the body is rarely far from 7.5, the 

 whole curve may be read from Fig. 15 provided merely the C0 2 



