66 RESPIRATORY MECHANISMS 



on the "respiratory center" in the medulla oblongata. In a 

 long-standing controversy it was debated whether C0 2 acted 

 as such or by virtue of its acid properties in solution. The 

 balance of evidence is definitely in favour of the specific action 

 of CO2 (M. Nielsen, 1936). The sensitivity of the center is 

 high (but, as we shall see later, subject to definite influences 

 from other parts of the nervous system). Breathing of C0 2 in 

 concentrations up to about 6% therefore greatly increases 

 ventilation, raising the alveolar C0 2 comparatively slightly. 

 At higher concentrations a narcotic and depressing influence 

 makes itself felt. The C0 2 effect has been demonstrated for a 

 number of air-breathing vertebrates belonging to all classes 

 (Vos, 1936), but strictly quantitative work has been done 

 mainly on man. 



The constancy of alveolar CO2 tension corresponding to a 

 constant production of the gas means that alveolar ventilation 

 must remain constant, while total ventilation may vary. This 

 is important for the temperature regulating function of respira- 

 tion. The expired air is saturated with moisture at a tem- 

 perature slightly below that of the body, and a considerable 

 amount of heat can be eliminated with water vapour by the 

 ventilation. By increasing the frequency and lowering the 

 depth of breathing total ventilation can be increased with a 

 constant low alveolar ventilation, because the ventilation of 

 the "dead space" of the air passages is proportional to the 

 frequency only. This is a mechanism for heat dissipation 

 utilized by many mammals and birds and often seen in dogs in 

 hot weather or after exertions. 



Respiration during muscular work in man. As emphasized on 

 p. 7, the performance of the respiratory organs during maxi- 

 mum sustained activity is the real test of their efficiency. 

 In man alone among animals the relation between muscular 

 activity and respiration is quantitatively well known. 



It is therefore appropriate to discuss conditions during work 

 in man. When increasing amounts of work are performed by 

 the same trained individual on an "ergometer," which meas- 



