RESPIRATION 199 



the regulation of alveolar and arterial CO 2 pressure resolves itself 

 into regulation of the blood reaction, and that the blood reaction 

 itself is a normal which is constantly being regulated within 

 marvelously narrow limits so narrow that the variations, evident 

 though they are made by physiological reactions, cannot be fol- 

 lowed adequately by existing physical and chemical methods. 



At this point it seems desirable to consider and criticize some of 

 the indirect means which have been used for estimating variations 

 in the hydrogen ion concentration of the blood. In recent years the 

 capacity of the blood, or of its serum, for combining with CO 2 has 

 commonly been taken as an index of hydrogen ion concentration, 

 this capacity being also alluded to as a measure of the "alkaline re- 

 serve" of the blood. It is evident that the "alkaline reserve" of the 

 blood is only another name for the "titration alkalinity" when CO 2 

 is allowed to escape. It is also evident from facts described above 

 that the alkaline reserve is increased in conditions of acute acidosis 

 due to excess of CO 2 , and diminished in conditions of acute al- 

 kalosis due to excessive lung ventilation caused by artificial res- 

 piration or anoxaemia. Hence although the alkaline reserve is 

 diminished in acidosis due to the presence of abnormal acids in 

 the blood, a diminution in alkaline reserve cannot be regarded as 

 by itself an index of acidosis. There is, in fact, no necessary con- 

 nection between diminution in alkaline reserve or titration alka- 

 linity and diminution in blood alkalinity. 



Another indirect method which has been used for estimating 

 variations in alkalinity is observation of one or more points in the 

 dissociation curve of the oxyhaemoglobin of the blood in presence 

 of the existing alveolar CO 2 pressure. This method is due to 

 Barcroft and his pupils, and is based on the following facts. ( I ) 

 As was shown in Chapter IV, each point in the dissociation curve 

 of oxy- or CO-haemoglobin in blood is simply displaced to a pro- 

 portional distance to the right or left on varying within wide limits 

 the partial pressure of CO 2 . Thus only one constant in the equa- 

 tion expressing the curve is altered. (2) It was shown by Peters, 48 

 and this had been completely confirmed by Hasselbalch, 49 that the 

 alteration in the constant depends, in cases where only the CO 2 

 pressure is varied, on alterations in the hydrogen ion concentra- 

 tion, and can thus be used as a measure of it. Barcroft and others 

 have therefore used the alteration in the constant as a measure in 

 all cases of variation of hydrogen ion concentration in the blood. 



48 Barcroft, The Respiratory Functions of the Blood, p. 316. 



49 Hasselbalch, Biochem. Zeitschr., 78, p. 132, 1916. 



