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MANUAL OF PHYSIOLOGY. 



therefore a less pressure exerted by that gas. This is shown by 

 the fact that the air given out by the latter half of a single ex- 

 piration has less O and more CO 2 than that of the first half. The 

 most impure air lies in the alveoli of the lung, for, since the tidal 

 air scarcely fills the tubes, the air in the alveoli is only changed 

 by mixture and diffusion with the impure air of the small bronchi. 

 Any impediment to the ordinary ventilation of the alveoli so 

 reduces the percentage, and therefore the tension of the O, that 

 it would probably sink below that in the blood, and in that case, 

 were it not a chemical union, the O would escape from the blood 

 in proportion as its tension in the blood exceeded that of the air 

 of the alveoli. We know it does not do this, even in the intense 

 dyspnoea of suffocation. 



In the same way the difference of tension of the CO 2 in the 

 alveolar air and in the blood, hardly explains the steady manner 

 in which the CO 2 escapes, and it has therefore been suggested 

 that this escape is also in some way a chemical process, possibly 

 connected with the union of the O and haemoglobin ; because the 

 admission of O to the blood seems to facilitate the exit of the 

 CO 2 . 



The following table gives the approximate tension of the two 

 gases in the different steps of the interchange, and shows that the 

 tensions are such as to enable physical absorption to take some 

 share in the entrance of the O as well as in the escape of the 

 CO 2 . A separate column gives the volumes per cent, of each gas, 

 corresponding to these tensions. This process must occur before 

 the oxygen and the haemoglobin meet, since the latter is bathed 

 in the plasma, and further separated from the O by the vessel 

 wall and epithelium. 



