184 PRACTICAL PHYSIOLOGY 



Carbon dioxide approximately -^- x 760 = 0'228 mm. of mercury 



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 XLVII. 

 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 continuous samples of the air expired 



FIG. 182. Zuntz respiration apparatus. The subject expires through the meter. 

 The inlet and outlet tubes are controlled by valves D and C, made of pieces of 

 intestine which have been soaked in glycerine. A small sample of the expired air is 

 steadily drawn off into the burette A by the escape of mercury from the tube which 

 is lowered by the revolution of the meter B. The meter gives the total volume of 

 air breathed. The measured sample in the burette is analysed by Haldane's gas 

 apparatus. 



through a meter. The method introduced by Zuntz for the purpose 

 of collecting such a sample is illustrated in Fig. 182. 



