CHAPTER XXXIX 



THE CONTROL OF RESPIRATION (Cont'd) 



THE ESTIMATION OF ALVEOLAR GASES 



By R. G. Pearce, B.A., M.D. 



Methods such as that of Haldane and Priestley, which calculate the 

 mean percentage composition of the alveolar air by analysis of a sample 

 taken from the end of a prolonged forced expiration, give values which 

 are too high for C0 2 and too low for 2 . There are several reasons 

 for this: (1) In the time taken for the prolonged deep expiration an 

 appreciable amount of CO, will be given off by the blood to the alveolar 

 air, and oxygen will be absorbed — that is, the sample will not contain 

 the same percentages of C0 2 and 2 at different stages of expiration. 

 (2) The portion of the tidal air which reaches the alveoli dilutes the 

 alveolar air and thus causes the amount of C0 2 given off by the blood to 

 vary during the different phases of respiration. If we bear in mind that 

 the tensions of CO, in the alveolar air and in the blood leaving the lungs 

 are always the same (page 343), and that the entire fall in C0 2 tension 

 in the alveolar air occurs during inspiration, then it is clear that the 

 blood in the pulmonary capillaries must have a maximum tension and 

 load of C0 2 at the end of expiration, and a minimum tension and load 

 of CO, at the end of inspiration. Accordingly, the average of the per- 

 centage of CO, and 0, at the end of inspiration and expiration, as de- 

 termined by the Haldane-Priestley method or by any of its modifications, 

 nn ist fail to give the correct mean tension of these gases in the alveolar 

 air during expiration. The error which makes the CO, higher than it 

 should be, makes the percentage of 0, less than it should be. These in- 

 fluences taken along with the fact, which will be shown later, that the 

 evolution of CO, from the blood is relatively more rapid at low than at 

 high tension of CO,, indicates that the blood in the pulmonary capil- 

 laries during inspiration must contribute a greater part of the CO, 

 excreted during a respiratory cycle than that in the pulmonary capil- 

 laries during expiration, and moreover that a greater part of the CO, 

 excreted must be evolved at a tension which is below the mean tension 

 of the CO, present in the entire time of the expiration. We conclude, 

 therefore, that the average tension of CO, in the alveolar air, determined 



344 



