144 PRACTICAL PHYSIOLOGY 



the residual and supplemental air (3500 c.c.). The composition of 

 the alveolar air is thus altered by only about one-twentieth at each 

 breath. These facts are determined by taking an inspiration from a 

 gasometer containing pure hydrogen, and analysing the amount of 

 hydrogen in the air expired. 



The Tension of Carbon Dioxide and of Oxygen in the Alveolar Air 

 of Man. Haldane determines the tension of the gases in the alveolar 

 air by an analysis of the last portion of the air expired in an ordinary 

 expiration. The experiment may be performed in the following way. 

 An . anaesthetic mask is connected by a T piece to a piece of tubing 

 80 cm. long and 1 '8 cm. internal diameter ; to the free end of the T 



Fio. 127. Apparatus for collection of a sample of Alveolar Air. 



piece is connected (Fig. 127) a gas-sampler with a capacity of 50 cubic 

 centimetres. The subject of the experiment fits the mask to his face 

 and makes an ordinary expiration ; as soon as the expiration ceases, 

 the tap of the gas-sampler, the air of which has previously been 

 removed by a vacuum-pump or gas-pump, is opened and a sample of 

 the last portion of the expired air is collected before the mask is 

 removed from the face. The analysis of the air is performed in the 

 manner already described. The percentage composition is about 5*5 

 carbon dioxide, 14*5 oxygen and 80 nitrogen. 



It is an advantage to determine the volume of each expiration by a 

 spirometer attached to the end of the tubing, and it is important 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. 



The respiratory exchange of gases can be determined in man by the 

 Haldane-Pembrey or Zuntz apparatus. In an average man weighing 



