CHAP, ii.] RESPIRATION. 575 



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. 



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 

 remark that at ordinary 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 (1*7000 feet) at which the pressure of the whole atmo- 

 sphere 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. 



356. The statements made so far refer to ordinary breathing, 

 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 oxygen in them 

 soon falls below the oxygen-pressure of ordinary venous blood. 

 But in such a case the blood is no longer ordinary 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 haemoglobin. 

 And as the reduction continues 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. 



The Exit of Carbonic Acid. 



357. It seems natural to suppose that the carbonic acid 

 would escape by diffusion from the blood of the alveolar capillaries 



