42 RESPIRATION 



abandoned if Krogh and Lindhard are correct. Their reasoning 

 is quite logical, but their premises are unsound. They have failed 

 to take into consideration trje anatomical relations of the air- 

 passage alveoli to the air-sac alveoli. 



The fact that the mixed air from all the air sacs of the lungs is 

 the same in composition however much of this air is expelled in 

 taking a sample led us to assume almost unconsciously that the 

 composition of the air in practically all the air sacs is the same. 

 Nevertheless all that the experiments prove is that the average 

 composition of the air expelled from the air sacs is the same, while 

 in individual air sacs the composition may vary widely. 



It is evident that in any particular air-sac system the mean 

 composition of the contained air will depend on the ratio between 

 the supply of fresh air and the flow of blood. If the supply of 

 fresh air is unusually small in relation to the supply of venous 

 blood there will be a lower percentage of oxygen and higher 

 percentage of carbon dioxide in the air of the air sac, and vice 

 versa. It seems probable that by some means at present unknown 

 to us a fair adjustment is maintained normally between air supply 

 and blood supply. For instance, the muscular walls of bronchioles 

 may be concerned in adjusting the air supply, or the arterioles 

 or capillaries may contract or dilate so as to adjust the blood 

 supply. In any case what seems to matter is the degree of arteri- 

 alization, not of the blood from individual air sacs, but of the 

 mixed arterial blood; and if the composition of the mixed air-sac 

 air served as a reliable index of the arterialization of the mixed 

 arterial blood we might dismiss as a matter of only academic 

 interest the question whether the air in individual air sacs varies 

 in composition. 



It will be shown below that there can be little doubt that under 

 normal conditions the air in different air sacs varies appreciably 

 in composition, and that under abnormal conditions the variation 

 may be considerable. It will also be shown that the latter fact 

 is one of great importance in pathology and therapeutics. 



Meanwhile it is clear from the experiments described in the 

 present chapter that under normal conditions, excluding heavy 

 work, the breathing in man is on an average regulated by the al- 

 veolar CO 2 pressure ; and a very slight increase or diminution in 

 the alveolar CO 2 pressure suffices to cause a very great increase or 

 diminution in the breathing. This conclusion has thrown a flood 

 of clear light on the physiology of breathing. 



