RESPIRATION 41 



It will be shown in Chapter IX that during rest under normal 

 conditions the gas pressures in the alveolar air and blood passing / 

 through the alveoli come into exact equilibrium. Now it has just j 

 been shown that in a very appreciable part of the lung alveoli 

 (those in the respiratory bronchioles, alveolar ducts, and atria) 

 the CO2 pressure is lower, and the oxygen pressure higher, than 

 in the air-sac alveoli. We might therefore be led to infer that in 

 the mixed arterial blood the CO2 pressure will be lower, and the 

 oxygen pressure higher, than in the blood from the air-sac alveoli, 

 and that in consequence of this the mixed arterial blood will have 

 a lower CO2 pressure than that of the deep alveolar air. Further 

 consideration shows, however, that this will not be the case. The 

 walls of the alveoli of respiratory bronchioles, etc., are in contact 

 on the one side with the air of air-passages, but on the other with 

 air in the air-sac alveoli. Hence the extra proportion of CO^ 

 extracted from the blood in the air-passage alveoli is practically 

 taken from the air-sac alveoli, and this is why the apparent respira- 

 tory quotient of the air-sac alveoli is lower than the true respira- 

 tory quotient. We should be counting the lowering twice if we as- 

 umed that in consequence of the extra discharge of CO2 in the re- 

 spiratory bronchioles, etc., the CO2 pressure of the arterial blood is 

 lower than corresponds to that of the air-sac alveoli. The same 

 argument applies also as regards the oxygen pressure of the air- 

 sac air, although under normal conditions hardly any extra oxy- 

 gen can pass into a given volume of blood in its passage through 

 the alveoli of respiratory bronchioles, etc. Hence the gas pressures 

 of the air-sac alveoli represent truly the mean gas pressures to 

 which the arterial blood is saturated in the various alveoli. This 

 is why the gas pressures of the deep alveolar air as determined by 

 the method which Priestley and I introduced are of so much 

 importance. 



Krogh and Lindhard^^ still maintain that the mean gas pres- 

 sures to which the blood is equilibrated in passing through the 

 lungs is given, not by the composition of the deep alveolar air, 

 but by that of the alveolar air as calculated from a fixed, or almost 

 fixed, dead space. This involves the conclusion that during deep 

 breathing, including the deep breathing of muscular exertion, the 

 arterial CO2 pressure is far lower than is shown by the direct 

 method of Priestley and myself. As, however, there is no cor- 

 responding apnoea, the whole theory of regulation of breathing in 

 accordance with the CO2 pressure of the arterial blood must be 



^ Krogh and Lindhard, Journ. of Physiol., LI, p. 59, 1917. 



