116 JRESPIRATION 



hyperpnoea or apnoea, as the case may be. Voluntary efiort, 

 therefore, though it may affect the respiratory movements 

 temporarily, does not affect the ultimate gaseous exchange. 

 The increased respiratory movements which accompany 

 a great need for oxygen, as in exercise, are brought about 

 involuntarily. 



In 1905 Haldane and Priestley showed conclusively that 

 the activity of the respiratory centre is influenced by the 

 composition of the blood supplying it. Their results may 

 be thus summarised — 



1. The partial pressure of COg in the alveolar air is 

 constant for each individual when in the resting state. 

 It is about 40 mm. 



2. The tension of COg in the blood leaving the lung is 

 equal to its pressure in the ah^eolar air. 



3. Any change induced in the pressure of COg in the 

 alveoh is transmitted to the arterial blood. 



4. "When COg is injected into the blood supplying the 

 medulla respiration is increased. 



5. A very shght rise of COg alveolar pressure causes 

 increased depth and rate of respiration. 



The chain of evidence is therefore complete that the extent 

 of pulmonary ventilation depends upon the tension of COg 

 in the arterial blood. 



Carbonic acid, however, is not the only substance which 

 affects the medulla. Any acid has a similar effect. The 

 responsible factor is now known to be the hydrogen ion 

 concentration of the blood. 



The question now arises whether the tension of oxygen 

 has also an effect upon the respiratory centre. Haldane 

 and Priestley found that the tension of oxygen had to be 

 very considerably diminished before any respiratory dis- 

 turbance was produced. When increased respiration does 

 occur under these circumstances, it is attributed not to 

 deficiency of oxygen directly, but to accumulation of acids 

 in the centres themselves owing to incomplete oxidation. 



The comparative indifference of the respiratory centre 



