CHAP, ii.] RESPIRATION. 465 



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 atmos- 

 pheric 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 observa- 

 tion 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 remark that at ordi- 

 nary altitudes, even taking into account the diminution the 

 oxygen undergoes before it reaches the pulmonary alveoli, the 

 partial pressure of the oxygen in the atmosphere leaves a wide 

 margin of safety. But at an altitude of 5500 metres (1700 feet) 

 at which the pressure of the whole atmosphere stands at about 

 the limit given above of 300 mm., the partial pressure of the 

 oxygen will be such that the venous blood cannot take up the 

 quantity of oxygen proper to convert it into arterial blood, since 

 at this limit arterial blood begins to give off oxygen. We may 

 add that it is at this altitude that breathing becomes especially 

 difficult, but to this we shall return. 



287. The statements made so far refer to ordinary breath- 

 ing, but the question may be asked, What happens when the 

 renewal of the air in the pulmonary alveoli ceases, as when the 

 trachea is obstructed ? In such a case the oxygen in the alveoli 

 is found to diminish rapidly, so that the partial pressure of oxy- 

 gen in them soon falls below the oxygen-pressure of ordinary 

 venous blood. But in such a case the blood is no longer ordi- 

 nary venous blood ; instead of being moderately, it is largely 

 and increasingly reduced ; instead of containing a comparatively 

 small amount, it contains a large and gradually increasing 

 amount, of reduced hsemoglobin. And as the reduction con- 

 tinues to increase, the oxygen-pressure of the venous blood also 

 continues to decrease ; it thus keeps below that of the air in the 

 lungs. Hence apparently even the last traces of oxygen in 

 the lungs may be taken up by the blood, and carried away to 

 the tissues. 



Guided by these observations then, we should be led to con- 

 clude that the film of the conjoined pulmonary and capillary 

 wall does not exert any influence, by virtue of its being a living 

 structure, upon the entrance of oxygen into the blood, or indeed 

 exert any influence at all even as a mere membrane or septum; 

 the oxygen appears to pass into the blood in the same way that 

 it would if the blood were freely exposed to the alveolar air 

 without any intervening partition. Nevertheless there are facts 

 which seem to throw doubt on the validity of this conclusion. 



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