200 RESPIRATION 



In persons at high altitudes, for instance, the constant is appar- 

 ently quite normal in presence of the existing alveolar COg pres- 

 sure; and from this fact it was inferred that the hydrogen ion 

 concentration of the blood is also normal, as already mentioned. 

 On the other hand, in persons who have shortly before undergone 

 some excessive muscular exertion the constant is very markedly 

 altered ; and from this fact a corresponding increase of hydrogen 

 ion concentration in the blood is inferred. The same method has 

 been employed for estimating variations of hydrogen ion con- 

 centration in the blood of patients. 



When, however, the facts are examined more closely it appears 

 that there must be a flaw in the reasoning. In the case of persons 

 who have completed some severe muscular exertion in a few min- 

 utes before, there is no physiological evidence of anything but a 

 most trifling acidosis, such as could not possibly be detected by 

 alterations in the constant of the dissociation curve. The breathing 

 is only increased to such an extent as to reduce the alveolar COo 

 pressure by about a fifth. This only corresponds to an acidosis 

 equivalent to what would be produced by a rise of 0.3 mm. in the 

 alveolar CO2 pressure ; and such a rise would be entirely inappreci- 

 able in its effect on the dissociation curve of oxyhaemoglobin. The 

 rise apparently indicated by the alteration in the constant is enor- 

 mously greater. Hence it appears that there must be some other 

 cause for the alteration than rise in hydrogen ion concentration. 

 This other cause is probably operative in many cases of patho- 

 logical acidosis. 



Another indirect method of measuring hydrogen ion concen- 

 tration has been proposed by Hasselbalch.^^ He showed quite 

 clearly that when the pressure of CO2 is varied in blood or even 

 serum the hydrogen ion concentration, as separately determined, 

 is proportional to the ratio of combined CO2 (which, as already 

 explained, is a measure of the bicarbonate present) to free COo 

 when allowance is made for the percentage ionization of the free 

 COo and bicarbonate. This corresponds with the fact that the blood 

 behaves as if more alkali were constantly being added to it in pro- 

 portion as its reaction approaches the neutral point. It is very re- 

 markable how closel}'- Hasselbalch's law holds for different kinds 

 of blood and in blood serum, in spite of great differences in the 

 dissociation curves for COs- Figure 58 from Hasselbalch's paper 

 is also very interesting as showing (for fresh ox blood) the dif- 

 ferences in the dissociation curves for serum, blood, and corpuscles. 



" Hasselbalch, Biochem. Zeitschr., 78, p. 112, 19 16. 



