Respiration 



173 



261. The Changes of Air in Breathing. The air which we 

 exhale during respiration differs in several important particulars 

 from the air we inhale. Expired air contains about 5 per 

 cent less oxygen and 5 per cent more carbon dioxide than 

 inspired air. 



The temperature of expired air is usually higher than that 

 of inspired air ; it is also loaded with aqueous vapor, imparted 

 to it in the upper air passages. The average daily quantity of 

 water exhaled as vapor is about one-half pint. 



The quantity of oxygen removed from the air by the breath- 

 ing of an adult person at rest amounts daily to about eighteen 

 cubic feet. About the same 

 amount of carbon dioxide is 

 expelled. 



Expired air contains, 

 besides carbon dioxide, a 

 small amount of poisonous 

 organic matter, which, from 

 the presence of micro- 

 organisms, introduced in the 

 inspired air, is apt to putrefy 

 rapidly. 



Various animal sub- 

 stances give rise, by de- 

 composition, to distinct 

 poisonous products known 

 as ptomaines. 



It is possible that some 

 of the constituents of the 

 expired air are of an allied 

 nature. At all events, these 

 substances have an injuri- 

 ous action, for an atmos- 

 phere containing simply i per cent of pure carbon dioxide has 

 very little harmful effect on the animal economy, but an atmos- 

 phere in which the carbon dioxide has been raised even half 

 of i per cent by breathing is highly injurious. 



NOTE. The nature, propagation, and action of bacteria are 

 described in more detail under the topic of " Bacteria," in Chapter 

 XIII of this book. 



FIG. 90. Capillary Network of the 



Air Sacs and Origin of the 



Pulmonary Veins. 



A, small branch of pulmonary artery ; B, twigs 

 of pulmonary artery anastomosing to form 

 peripheral network of the primitive air cells ; 

 C, capillary network around the walls of the 

 air sacs ; D, branches of network converg- 

 ing to form the veinlets of the pulmonary 

 veins. 



