SEC. 3. CHANGES OF THE AIR IN RESPIRATION. 



337. 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 consequence 

 the expired air differs from inspired air in several important 

 particulars. 



The temperature of expired air is variable, but under ordinary 

 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 ex- 

 pired air sinks somewhat, but not to any great extent, thus at 

 -6'3C. 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 3S'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. 



338. 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. 



339. The expired air contains about 4 or 5 p.c. less oxygen, 

 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 7915 '04 



Expired 16'033 79 "587 4'38 



