330 EESPIRATION 



only about thirty cubic inches is inhaled and exhaled in a 

 single respiration. This is called tidal air. The one hundred 

 cubic inches left in the lungs after the fullest possible 

 expiration is called residual air. 



The significance of the small amount of tidal air and 

 the presence of a certain constant amount of residual air 

 is found in the fact that such a condition allows the blood 

 exchange to take place continually instead of at intervals 

 as would be necessary if the lungs were entirely emptied 

 at each expiration. Further, the possibility of var3dng 

 the amount of air content permits the lungs to accommo- 

 date the air supply to the demand of the tissues, that is, 

 when exercising the increased amount of oxygen needed 

 is met by deeper inspiration and expiration. 



Hygiene of Breathing. 



We have already noted that in order that the air be 

 properly warmed and cleared of dust before entering the 

 lungs it should pass through the nasal cavities rather 

 than through the mouth. We have also seen that the 

 action of the lungs is dependent upon the enlargement of 

 the chest cavity, and hence tight clothing or other inter- 

 ference with such enlargement is necessarily injurious. 



Like muscles, too, the capacity of the lungs and their 

 size are increased by use. Exercise increases both the 

 blood rate and the breathing rate. Now, in quick, short 

 breathing, only the air in the upper part of the lungs is 

 changed, while the increased rate of blood flow shoots the 

 corpuscles through the capillaries so rapidly that they do 

 not have time to effect a satisfactory exchange of oxyo-cn 

 and carbon dioxide unless there is a large and constant 



