150 ELEMENTARY PHYSIOLOGY less. 



blood and the fresh tidal air — who desire to exchange 

 their commodities, carbonic acid for oxygen, and oxygen 

 for carbonic acid. 



Now there is nothing interposed between the fresh tidal 

 air and the stationary air ; they are gaseous fluids, in 

 complete contact and continuity, and hence the exchange 

 between them must take place according to the ordinary 

 laws of gaseous diffusion. 



Thus, the stationary air in the air-cells, or, as it is ' 

 frequently called, Alveolar Air, gives up oxygen to the 

 blood, and takes carbonic acid from it, though the exact 

 mode in which the change is effected is not thoroughly 

 understood. By this process it becomes loaded with 

 carbonic acid, and deficient in oxygen. There is very 

 much greater excess of tlie one, and deficiency of the 

 other, than is exhibited by inspired air, seeing that the 

 latter acquires its composition by diffusion in the short 

 space of time (four or five seconds) during which it is in 

 contact with the alveolar air. 



Dry alveolar air contains in each 100 volumes — 



Oxygen Nitrogen Carbonic .-cid 



14-5 80 rro 



7. TherUhanges of Air in Respiration.— Expired air 

 differs from the air inspired in the following particulars. 



(i) Speaking generally, whatever be the temperature of 

 the external air, that expired tends to be nearly as hot as 

 the blood, or has a temperature of about 37' C. (98*6° F.). 



(ii) However dry the external air may be, that expired 

 is nearly, or quite, saturated with watery vj^our. This 

 vapour is not derived from the stationary air, but from 

 the walls of the outer aii' passages, so that the inspired 

 air is practically saturated with aqueous vapour before it 

 reaches the bronchi. 



(iii) While ordinary inspired air contains in 100 

 volumes — 



Oxygen Niti-ogeu Carbonic Acid 



20-96 7900 04 



