THE CHEMISTRY OF RESPIRATION 1173 



tidal air amounts to 500 c.c. If he breathes seventeen times a minute 

 the total pulmonary ventilation during the hour will be 500 x 17 x 60 

 = 510,000 c.c. per hour. This will contain 300 X 70 c.c. = 21,000 c.c. 

 carbon dioxide. Hence the percentage of carbon dioxide in the 

 expired air will be 4*1 per cent. In the same way we can reckon the 

 percentage of oxygen in the expired air at 16- 4 per cent. Exact 

 experiments have shown that the volume of nitrogen is unchanged 

 during respiration, this gas taking no part in the ordinary metabolic 

 processes of the body. We may therefore compare the ordinary 

 composition of inspired and expired air as follows : 



INSPIRED AIR 



Oxygen. ..... 20-96 vols. per cent. 



Nitrogen and argon . . . 79-00 

 Carbon dioxide .... 0-04 



EXPIRED AIR 



Oxygen. ..... 16-4 vols. per cent. 



Nitrogen ..... 79-5 



Carbon dioxide . . . . 4-1 ,, 



The increase in the figures for nitrogen refers of course only to the 

 percentage amount, since the total volume of air breathed is decreased 

 by the disappearance of a certain amount of oxygen without the pro- 

 duction of a corresponding amount of carbon dioxide, so that the 

 relative amount of nitrogen is slightly increased. These figures for 

 the composition of inspired and expired air refer to dry air at a tempera- 

 ture of C. and a pressure of 760 mm. Under normal circumstances 

 inspired air contains a variable amount of aqueous vapour and has 

 a variable temperature corresponding with the time of year. Expired 

 air is fully saturated with aqueous vapour and has the temperature 

 of the body, 37 C. The aqueous vapour at this temperature is by 

 no means negligible. Its tension amounts to 50 mm. Hg. Thus 

 when a man is breathing dry air at a pressure of 760 mm. Hg., the 

 pressure of the mixture of gases in the alveoli of his lungs will be only 

 760 50, i.e. 710 mm. Hg. 



Only a certain percentage of the 500 c.c. of tidal air reaches the 

 alveoli, 100 to 140 c.c. being required to fill the trachea and bronchial 

 tubes. Hence the alveolar air must contain more carbon dioxide 

 and less oxygen than the tracheal air ; and it is found that, if we take 

 the air from the alveoli instead of that expired through the mouth or 

 nose, the differences between it and the inspired air are much more 

 pronounced. 



A sample of alveolar air may be obtained for analysis in the following way 

 (Haldane) : A piece of india-rubber tubing is taken of about 1 inch diameter 

 and 4 feet long. Into one end (Fig. 490) is fitted a mouthpiece, the other being 



