IV CHANGES OF AIR IN RESPIRATION 151 



the composition of expired air is on the average in 100 

 volumes — 



Oxygen Nitrogen Carbonic Acid 



16-50 79-50 4-00 



Thus, speaking roughly, air which has been breathed 

 once has gained 4 per cent, of carbonic acid and lost rather 

 more than 4 per cent, of oxygen, the quantity of nitrogen 

 being practically unchanged. 



(iv) Expired air contains, in addition, small quantities 

 of "animal matter" or organic impurities of a highly 

 decomposable kind. Nothing is known of their nature, 

 but they are probably the chief cause, why air which has 

 been breathed once is extremely unwholesome if breathed 

 a second time ; hence they are of great importance in con- 

 nection with ventilation (see p. 168). 



(v) The volume of the tidal air is but little altered by 

 being breathed, because the two parts of oxygen in the 

 carbonic acid (CO.,) occupy the same volume as the car- 

 bonic acid itself, or in other words the volume of the 

 carbonic acid is equal to that of the oxygen contained in 

 it. But as a matter of fact very close analysis of the 

 expired air shows firstly that the volume of oxygen 

 which disappears is slightly greater than the volume 

 of carbonic acid which takes its place. This is because 

 all the oxygen taken in does not go to form car- 

 bonic acid ; some of it unites with hydrogen to form 

 water and some with other elements such as sulphur. 

 Hence the volume of the expired air is slightly (^q) less 

 than that of the inspired air. In the second place careful 

 analysis shows that the nitrogen in expired air may vary 

 very slightly : sometimes it is a little in excess of, some- 

 times slightly less than, that inspired, and sometimes it 

 remains unaltered. 



8. The Amount ofWaste which leaves the Lungs.— 

 About 10,000 litres (from 350 to 400 cubic feet) of air are 

 passed through the lungs of an adult man taking little or 



