114 



RESPIRATION 



in and out at each respiratory excursion being smaller than 

 the total capacity of the lungs, a complete interchange 

 between the atmosphere and the lungs does not take 

 place at each respiration. The result of this is that the 

 temperature of the entering air is raised by that of the 

 outgoing air, so that the former is almost at body tempera- 

 ture by the time it reaches the alveoli. 



The walls of the bronchi are held open by the pull upon 

 them of the elastic lung tissue, in antagonism to which 

 are the constrictor muscles of the bronchi, which tend to 

 keep the passages shut. The bronchial muscles are under 

 the control of the vagus, which exerts a constant tonic 

 influence over them. On stimulation of the peripheral 

 end of the vagus these muscles are contracted. 



During inspiration the passages are dilated by the 

 increased pull of the lung tissue, and during expiration 

 slightly constricted. In asthma the bronchial muscles 

 undergo spasmodic contractions. The patient therefore 

 makes violent inspiratory efforts to keep the tubes open. 



The Exchange of Gases between the Lungs and the 



Atmosphere 



The composition of inspired air, expired air and alveolar 

 air is here shown, excluding water vapour, w^th which 

 expired and alveolar air are saturated. 



The following are the volume changes induced — 

 The volume breathed in normal inspiration = 300- 

 500 c.c. {Tidal air). 



The volume which can be inhaled by an effort, super- 



