1102 PHYSIOLOGY 



similar sample is then taken, in which the subject expires deeply at the end of a normal 

 expiration. This sample will, of course, contain more C0 2 and less O 2 than that obtained 

 at the end of inspiration. The mean of the two samples is taken as the average com- 

 position of alveolar air. 



The difference between the composition of expired air and alveolar air 

 is determined by the dilution of the alveolar air with that contained in the 

 dead space. Hence with shallow breathing there will be a large difference, 

 but this will decrease with increased depth of respiration. Thus, if the 

 alveolar air contained 6 per cent. CQ 2 and the dead space amounted to 

 150 c.c., the expired air would contain only 3 per cent. C0 2 when the person 

 was taking in only 300 c.c. at each respiration. If however he was breath- 

 ing slowly and deeply so as to raise the tidal air to 1500 c.c., only one-tenth 

 of this would be represented by the dead space, and the expired air would 

 contain nine-tenths as much C0 2 as the alveolar air, i. e. 54 per cent. 



TUBE. 



FIG. 504. 



The changes in the composition of alveolar air with respiration are by 

 no means so marked as those produced in the tidal air, since the latter 

 forms only a small proportion of the total air in the lung alveoli. Thus 

 at the end of a normal expiration the alveoli still contain 2500 c.c. of gases. 

 In inspiration 360 c.c. atmospheric air is taken into this space and mixed 

 with the 2500 c.c. already there. The ( ventilation coefficient ' in quiet 

 breathing is therefore only one-seventh, and the change in the oxygen and 

 carbon dioxide content of the alveolar air produced by this access of 360 c.c. 

 will amount to less than one-half per cent. This is illustrated by the follow- 

 ing figures from Haldane, giving the alveolar content in carbon dioxide at 

 the end of inspiration and at the end of expiration respectively. 



ALVEOLAR C0 2 TENSIONS 



We can thus speak of an average composition of alveolar air which, in 

 spite of the constant ventilation, differs from the external air in containing 

 an excess of carbon dioxide and a relative lack of oxygen. Lavoisier, who 

 was the first to study the chemical changes in respiration accurately, regarded 

 the lungs as the seat of the formation of carbon dioxide and the consump- 

 tion of oxygen. This view was generally accepted until it was shown by 



