182 PRACTICAL PHYSIOLOGY 



burette are kept moist, variations in the tension of aqueous vapour in 

 the burette are also corrected by the control tube. 



A sample of expired air is obtained by- breathing through the tube 

 into the burette B (Fig. 180). A and B are filled with acidulated 

 water, and B is controlled by a clip. 



The portion of B which lies beyond the clip is squeezed empty of air 

 before it is inserted over the entrance tube of the Haldane gas-burette. 

 The sample is then taken over by lowering the levelling tube and 

 opening the clip. 



Atmospheric Air, measured dry at standard temperature and 

 pressure, and 760 mm., has the following composition : 



Oxygen, - 20'94 volumes per cent. 



Carbon dioxide, - 0'03 



Nitrogen, - . 78-09 



Argon, 0-94 



There are also traces of helium, krypton, neon, xenon, and hydrogen. 

 The nitrogen and argon appear to be inert as far as the higher animals 

 are concerned, and in ordinary analyses are given together as nitrogen. 



The Expired Air varies in composition according to the rate and 

 depth of respiration ; this is shown by the following analyses made by 

 Speck. 



Volume of air expired 



Type of per minute. Percentage of Percentage of 



breathing. c.c. oxygen. carbon dioxide. 



Normal, - 7,527 16-29 4-21 



Very shallow, - 5,833 15'50 4'63 



Very deep, - 17,647 18'29 3'17 



Stated in whole numbers the composition may be given as follows : 



Volumes per cent. 



Oxygen. Carbon dioxide. Nitrogen. 



Inspired air, 21 (0'03) 79 



Expired air, 16 4 80 



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. 



