DIFFUSION IN THE LUNGS 149 



been occupying the upper air passages where no interchange 

 occurs. The bronchial capacity is only about one-third large 

 enough to accommodate the tidal air, and consequently the 

 greater part of it must come from lower down in the lung 

 structure, and the CO2 in the expired air continuously in- 

 creases until the end of the act. At each inspiration at least 

 two-thirds of the tidal air must pass into the small bronchi, 

 or lower. Thus it is that inspiration and expiration them- 

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

 least the bronchioles) air fresh with O and air vitiated 

 with CO2, aid very materially in keeping constant the com- 

 position of the alveolar air. 



In the second place, the cardiac movements have a similar 

 effect, each systole decreasing the size of the heart and in- 

 ducing 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 

 physical 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 atmos- 

 pheric conditions, exerts a certain pressure. In every me- 

 chanical mixture of gases (such as the atmosphere) each in- 

 dividual gas exerts a part of the total pressure a part pro- 

 portional 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 atmosphere 

 is 21 /ioo 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 pres- 

 sure in that situation is less than in the trachea and bronchi. 

 The result is that O continually makes its way from the 

 point of higher pressure (trachea and bronchi) toward the 

 point of lower pressure (alveoli). The tendency is thus to 

 establish a uniform partial pressure throughout the whole 



