210 APPLIED PHYSIOLOGY 



vapour; but so, probably, is the inspired air before it 

 reaches the alveoli, thanks to the moistening influence 

 of the upper air passages already described. 



A comparison of the composition of inspired and 

 expired air does not, however, give us an accurate idea 

 of the actual composition of the air when it comes into 

 relation with the blood in the alveoli, for the following 

 reason : The air in the passages from the nose to the 

 alveoli is not really changed during respiration, but 

 comes out again the same in composition as it went in. 

 The air which has actually been in the alveoli, therefore, 

 is, as it were, diluted by this purer air when the total 

 output of a breath is collected, and the latter does not, 

 therefore, really represent the composition of the air as 

 it actually left the alveoli themselves. Seeing that the 

 volume of air in the air passages is about 140 c.c., and 

 the total air taken in and sent out again at one breath 

 is about 500 c.c., it will easily be seen that a considerable 

 fallacy is introduced if the composition of the expired 

 air be taken as representing that of the alveoli. As a 

 matter of fact, if the alveolar air be collected separately, as 

 it can be, it is found to contain much more C0 2 and less 

 oxygen than ordinary expired air, the oxygen amounting 

 to only 13 or 14 per cent., and the C0 2 to 5 or 6 per 

 cent. Further, this method of analysis has shown that 

 the partial pressure of C0 2 in the alveoli is constant. 



It will be seen from this that the essential fact of 

 respiration is that the blood gives up carbonic acid to 

 the air in the lung and receives back oxygen from it. 



When one comes, however, to ask how this exchange 

 is effected, one finds oneself involved in a maze of con- 



