RESPIRATION 183 



blood is rendered distinctly acid these buffer substances still act 

 very efficiently. The haemoglobin acts as an alkali, whereas it 

 always acts as an acid in blood within the living body. 



In order to test whether it is really to difference in PH that the 

 respiratory center normally reacts, Hasselbalch made the experi- 

 ment of altering the resting alveolar CO 2 pressure by changing 

 the diet. A meat diet, consisting largely of proteins containing 

 sulphur and phosphorus which break down into free sulphuric and 

 phosphoric acid, is evidently an acid-forming diet as compared 

 with a vegetable diet, which contains less protein and a relative 

 abundance of salts of organic acids which break up in the body so 

 as to yield carbonates. Hasselbalch found that with the acid meat 

 diet the resting alveolar CO 2 pressure was 4.4 mm. lower, and 

 then proceeded to compare the PH of the blood in the two condi- 

 tions. The results were as follows : 21 



It will be seen that at 40 mm. CO 2 pressure the blood sample 

 taken with the meat diet was distinctly more acid than with the 

 vegetable* diet, but that at the existing alveolar CO 2 pressure the 

 two values for PH were identical, at least within the limit of ac- 

 curacy of the method of measurement. Hence the respiratory 

 center had regulated the alveolar CO 2 pressure in such a manner 

 as to keep the PH of the blood almost constant. 



There is other evidence pointing in the same direction. Barcroft 

 found that on the Peak of Teneriffe the dissociation curve of 

 human blood appeared to be normal, provided that the curve 

 was investigated, not at the normal sea level alveolar CO 2 pressure 

 of about 40 mm., but at the existing resting alveolar CO 2 pres- 

 sure. 22 We got a similar result at a greater height on Pike's 

 Peak, 23 as did also Barcroft and his co-workers on Monte Rosa. 24 



21 Hasselbalch, Btochem. Zeitschr., XLVI, p. 416, 1912. 



22 Barcroft, Journ. of Physiol., XLII, p. 44, 1911. 



23 Douglas, Haldane, Henderson, and Schneider, Phil. Trans. Roy. Soc., (B) 

 203, p. 201, 1913- 



24 See Chapter XVII, of Barcroft, The Respiratory Function of the Blood, 1913. 



