ADVANCED EXPERIMENTAL PHYSIOLOGY 137 



Stated in whole numbers the composition may be given as 

 follows : 



There are other differences between inspired and expired air. 

 Under ordinary conditions expired air is warmed nearly to the 

 temperature of the body and is saturated with water vapour ; it 

 has about 6 per cent, of moisture, whereas ordinary atmospheric 

 air has about 1 per cent. 



The expired air is a mixture of air from the so-called " dead 

 space " of the respiratory tract and of air from the alveoli of the 

 lungs, where the exchange of gases between the blood and the air 

 takes place. The " dead space " extends from the nose to the alveoli 

 and has a capacity of about 150 c.c. in an adult man. In an ordinary 

 expiration the first portion of air to leave the nose or mouth is from 

 this " dead space," then mixed air, and finally air from the alveoli. 



The Alveolar Air. The composition of the alveolar air is deter- 

 mined, according to the method introduced by Haldane and 

 Priestley, 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-piece is connected (Fig. 134) a gas-sampler 

 with a capacity of 50 cubic centimetres. The subject of the ex- 

 periment 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. By 

 placing an " anaesthetic " rubber bag at the free end of the tube 

 it is easy to determine whether the subject has made an adequate 

 expiration. 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. 



