SEC. 2. CHANGES OF THE AIR IN RESPIRATION. 



268. During its stay in the lungs, or rather during its stay 

 in the bronchial passages, the tidal air (by means of diffusion 

 chiefly) effects exchanges with the stationary air; in conse- 

 quence the expired air differs from inspired air in several impor- 

 tant particulars. 



The temperature of expired air is variable, but under ordi- 

 nary circumstances is higher than that of the inspired air. 

 At an average temperature of the atmosphere, for instance at 

 about 20 C., the temperature of expired air is, in the mouth 

 33-9, in the nose 35-3. When the external temperature is 

 low, that of the expired air sinks somewhat, but not to any 

 great extent, thus at 6*3 C. it is 29 '8 C. When the external 

 temperature is high, the expired air may become cooler than 

 the inspired, thus at 41-9 it has been found to be 38-1. The 

 expired air takes its temperature from that of the body, that 

 is, of the blood, and this as we shall see later on while generally 

 higher may, at times, be lower than that of the atmosphere. 

 The exact temperature of the expired air in fact depends on 

 the relative temperatures of the blood and inspired air, and on 

 the depth and rate of breathing. The change in temperature 

 takes place not in the lungs but in the upper passages, and 

 chiefly in the nose and pharynx. 



269. The expired air is loaded with aqueous vapour. The 

 point of saturation of any gas, that is, the utmost quantity of 

 water which any given volume of gas can take up as aqueous 

 vapour, varies with its temperature, being higher with the 

 higher temperature. For its own temperature expired air is, 

 according to most observers, saturated with aqueous vapour. 

 The moisture, like the warmth, is imparted not in the depths 

 of the lung but in the upper passages. The inspired air as it 

 passes into the bronchia is already saturated with moisture. 



270. The expired air contains about 4 or 5 p.c. less oxy- 

 gen, and about 4 p.c. more carbonic acid than the inspired air, 

 the quantity of nitrogen suffering but little change. Thus 



oxygen. nitrogen. carbonic acid. 



Inspired air contains 20-81 79-15 -04 



Expired 16-033 79-587 4_i38 



440 



