216 PHYSIOLOGY FOR DENTAL STUDENTS. 



1500 c. c. more air into the lungs. This amount is known as the 

 complcmental air. Likewise after a normal expiration about 1500 

 c. c. more air can be expelled from the lungs. This is known as 

 the supplemental air. In spite of forced expiration there will 

 still remain within the lungs about 2000 c. c. of air which fills the 

 alveoli and air tubes, known as the residual air. This air remains 

 in the air spaces after the forced expiration because the lungs 

 cannot relax to their fullest extent, being held open by the suc- 

 tion pressure of the thorax. In other words, the thoracic cavity 

 is larger in the expiratory position by 2000 c. c. than the lungs 

 are. That this is the case is shown by the immediate contraction 

 of the lungs into a small volume when the thorax is opened, for 

 then the atmospheric pressure comes to be equalized on the out- 

 side and inside of the lungs, and the elastic tissue contracts and 

 forces out the residual air. From this it is obvious that the elas- 

 tic recoil of the stretched lungs must always tend to pull the 

 organ away from the chest wall and thus create a negative or suc- 

 tion pressure within the thoracic cavity. Anything which de- 

 stroys this relation makes breathing impossible, because the lungs 

 are no longer held against the chest walls. It is for this reason 

 that wounds in the chest are very dangerous. 



The trachea, bronchi, etc., require quite a little air to fill them, 

 so that only a part of the tidal air reaches the alveoli. In other 

 words, it is only a portion of the air we expire that has really 

 been in contact with the respiratory epithelium and has suffered 

 any change in composition. It is estimated that about 140 c. c. 

 represents the actual volume of the air tubes. 



This leaves 360 c. c. of air which reaches the alveoli. This 

 amount is used to dilute the 2000 c. c. of residual air and 1500 

 c. c. of supplemental air already in the alveoli. In fact the 

 function of breathing may be said to consist in continually dilut- 

 ing the alveolar air with a quantity of fresh air in order that its 

 composition may remain more or less constant. 



The above analysis shows that there is a marked difference be- 

 tween the inspired and the expired air. It shows us further that 

 of the oxygen taken up by the blood, only part appears again 

 combined with carbon in the gas C0 2 . The retention of oxygen 



