CHAP. IL] RESPIRATION. 605 



temperature of the body for instance. When the atmosphere in the 

 chamber contains a certain per-centage of the gas in question, that gas 

 is now neither given off nor taken up by the blood ; this indicates the 

 partial pressure of that gas in the blood. 



Without going into detail, we may state that these experi- 

 ments shew that the partial pressure of oxygen in the lungs 

 is amply sufficient to bring about, at the temperature of the 

 body, the association of that additional amount of oxygen by 

 which venous blood becomes arterial. When a solution of 

 haemoglobin or when blood is successively exposed to increasing 

 oxygen pressures, as the partial pressure of oxygen is gradually 

 increased, the curve of absorption rises at first very rapidly but 

 afterwards more slowly ; that is to say, the later additions of 

 oxygen at the higher pressures are proportionately less than the 

 earlier ones at the lower pressures. And this is consonant with 

 what appears to be the fact that the haemoglobin of arterial blood 

 though nearly saturated with oxygen, i.e. associated with almost 

 its full complement of oxygen, is, under ordinary circumstances, not 

 quite saturated. When arterial blood is thoroughly exposed to 

 air it takes up rather more than 1 vol. p.c. of oxygen ; and that 

 appears to represent the difference between exposing blood to pure 

 air, such as enters or ought to enter the mouth in inspiration, and 

 exposing blood to the air as it exists in the pulmonary alveoli. 

 The greater relative absorption at the lower pressures has a 

 beneficial effect in as much as it still permits a considerable 

 quantity of oxygen to be absorbed even when the partial pressure 

 of oxygen in the air in the lungs is largely reduced, as in ascending 

 to great heights. Further, other experiments seem to shew that 

 when the partial pressure of the oxygen in the air of the lungs 

 is increased beyond the normal, by making the animal breathe 

 oxygen, the partial pressure of the oxygen increases correspondingly ; 

 the haemoglobin becomes quite saturated with oxygen, and a 

 further quantity, small in amount but of high pressure, is taken 

 up by the plasma. 



Observations made both with dog's blood and ox's blood seem 

 to shew that arterial blood ceases to take up oxygen and begins to 

 give off oxygen, in other words, that dissociation begins to take 

 place, when the partial pressure of the oxygen in the atmosphere 

 to which it is exposed sinks to about 60 mm. of mercury, that is 

 to say, when the whole atmospheric pressure is reduced from 

 760 mm. to about 300 mm. or when the percentage of oxygen 

 in the atmosphere is reduced by decidedly more than half. And 

 this accords with the observation that, in man, when the oxygen 

 of inspired air is gradually diminished, without any other change 

 in the air, symptoms of dyspnoea do not make their appearance 

 until the oxygen sinks to 10 p.c. in the inspired air and must 

 therefore be less than this in the pulmonary alveoli. We may 



