READJUSTMENTS OF REGULATION 37 



tions between hydrogen ion concentrations and C0 2 

 pressure in blood. A difference in C0 2 pressure which 

 would be sufficient to double the breathing, or to cause 

 apnoea, produced a difference in hydrogen ion concen- 

 tration which was just measurable by the method, so 

 the method is very rough as compared with the deli- 

 cacy of discrimination by the respiratory centre. By 

 varying the diet from alkaline to acid-producing 

 Hasselbalch succeeded in producing a variation of 

 several millimetres in the alveolar C0 2 pressure. He 

 then found that with the blood saturated with C0 2 at 

 the existing alveolar C0 2 pressure the hydrogen ion 

 concentration as measured was sensibly the same on 

 either diet ; whereas if the blood was saturated in both 

 cases at the same C0 2 pressure the hydrogen ion con- 

 centration was markedly different on the two diets. 

 The difference in alveolar C0 2 pressure was thus just 

 sufficient to keep the hydrogen ion concentration, in so 

 far as it could be measured by the electrometric 

 method, constant in the two samples of blood, although 

 there was presumably a slight difference as indicated 

 by the difference in the breathing. Other similar ex- 

 periments had a similar result, and there seems now 

 to be no doubt that it is true that what the respiratory 

 centre responds to is hydrogen ion concentration, and 

 not mere C0 2 pressure. 



The delicacy of the response of the respiratory 

 centre to change in the reaction of the blood is very 

 extraordinary ; but what is still more marvellous is the 

 fact that in spite of this delicacy the alveolar C0 2 

 pressure is so steady during rest. The respiratory 



