1052 PHYSIOLOGY 



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 164 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 figure for nitrogen refers of course only to the per- 

 centage amount, since the total volume of air breathed is decreased by the 

 disappearance of a certain amount of oxygen without the production 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 temperature 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. 492) is fitted a mouthpiece, the other being left open or connected with a 

 spirometer. About 2 inches from the mouthpiece is fixed a gas sampling-bulb, which is 

 provided with three-way taps at the upper and lower ends. Before an experiment the 

 bulb is filled with mercury, if the lower end is open, or else it is completely exhausted. 

 The subject of the experiment, after breathing normally a few times, at the end of a 

 normal inspiration puts his mouth to the tube, expires quickly and deeply, and closes 

 the mouth-piece with his tongue. The tap of the sampling-bulb is then turned, and the 

 air last expelled from the lungs (which is therefore pure alveolar air) rushes into the bulb. 

 The tap of the bulb is then turned off, and the gas may be removed for analysis. A 

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





