DIFFUSION IN THE LUNGS 149 



tion at least two-thirds of the tidal air must pass into the small 

 bronchi, or lower. Thus it is that inspiration and expiration 

 themselves, taking into and bringing out of the vesicles (or at 

 least the bronchioles) air fresh with O and air vitiated with 

 CO 3 , aid very materially in keeping constant the composition of 

 the alveolar air. 



In the second place, the cardia'c movements have a similar effect, 

 each systole decreasing the size of the heart and inducing a fresh 

 atmospheric current toward the deep alveoli, and each diastole 

 forcing a like current of vitiated air toward the trachea. This 

 force is not inconsequential. 



In the third place, the diffusibility of gases under known phys- 

 ical laws, without the aid of any such movements as have been 

 described, is an occurrence in connection with the phenomenon 

 in question. Every gas, under ordinary atmospheric conditions, 

 exerts a certain pressure. In every mechanical mixture of gases 

 (such as the atmosphere) each individual gas exerts a part of 

 the total pressure a part proportional to its percentage in that 

 mixture. This has been called the "partial pressure" of that 

 gas. Since O is present in ordinary atmosphere to the extent of 

 21 parts per hundred, the partial pressure of oxygen in the at- 

 mosphere is i^ of the total pressure. 



Now, in the air of the alveoli O is present to a less extent than 

 21 parts per hundred, and consequently its partial pressure in 

 that situation is less than in the trachea and bronchi. The re- 

 sult is that O continually makes its way from the point of higher 

 pressure (trachea and bronchi) toward the point of lower pres- 

 sure (alveoli). The tendency is thus to establish a uniform 

 partial pressure throughout the whole respiratory tract; but 

 this is never done during life because the partial pressure above 

 is being continually increased by the introduction of new O, 

 and below is being continually diminished by the removal of 

 that gas from the alveoli by the blood. 



In case of CO 2 opposite conditions prevail. This gas is being 



