138 PRACTICAL PHYSIOLOGY 



It is an advantage to determine the volume of each expiration 

 by a spirometer attached to the end of the tubing, and it is impor- 

 tant that the subject of the experiment should by a little practice 

 with the apparatus learn to breathe naturally, otherwise a fair 

 sample will not be obtained. 



to Anaesthetic 

 .Rubber Bag 



FIG. 134. Apparatus for collection of a sample of alveolar air. 



The partial pressure, or, as it is often called, the tension of the 

 component gases is : 

 Dry atmospheric air : 



21 



Oxygen approximately X 760 = 159-6 mm. of mercury or 21 

 100 



per cent, of an atmosphere. 



79 



Nitrogen approximately - x 760 = 6004 mm. of mercury or 

 100 



79 per cent, of an atmosphere. 



Carbon dioxide approximately x 760 = 0-228 mm. of mer- 



100 



cury or 0-03 per cent, of an atmosphere. 



The tensions of the gases of the alveolar air are calculated in a 

 similar way, but the tension of aqueous vapour must be deducted 

 from the pressure of the atmosphere. 



CHAPTER XXII 



DETERMINATION OF THE RESPIRATORY EXCHANGE 



IN MAN ! 



An estimation of the intake of oxygen and output of carbon 

 dioxide can be made by analyses of samples of the air expired into 

 a Douglas bag. Collect a sample of expired air and analyse it ; 

 then determine by means of a meter the volume of air breathed in 



1 For further details see " The Methods for the Estimation of General 

 Metabolism," Part II. 



