262 RESPIRATION 



recondite hypothesis, until the necessity for doing so is clearly 

 demonstrated. 



It is unfortunate that complete unanimity has not been attained 

 on this question in regard to the oxygen absorption, for the avail- 

 able differences of partial pressure between air and blood are much 

 greater than in the case of the carbon dioxide, and were it definitely 

 shown that the process is a physical one for oxygen, there would be 

 little chance that it could be anything else for carbon dioxide. A 

 glance at the table on p. 259 shows that while the carbon dioxide 

 tension of venous blood may sometimes, perhaps generally, exceed 

 that of the alveolar air, the difference is quite small. The average 

 for the observations on man with the pulmonary catheter was 

 45 mm., which compares with an average alveolar tension of 42 mm. 

 If this excess of 3 mm. in favour of the blood be taken to show, as 

 it certainly could be, if the difference were a constant one, that 

 carbon dioxide can diffuse from the venous blood, as it enters the 

 pulmonary capillaries, into the air of the alveoli, the marked de- 

 ficiency in the carbon dioxide tension of arterial blood ought to be 

 interpreted as meaning that diffusion is not the only way in which 

 the blood gets rid of its carbon dioxide in making the round of the 

 pulmonary circulation. 



In Bohr's experiments, in some of which the animals were made 

 to breathe air containing carbon dioxide in various proportions, the 

 tension of that gas in the alveolar air was often greater than in the 

 arterial and even than in the venous blood, and yet carbon dioxide 

 was given off by the blood to the lungs. 



It does not seem improbable in itself that the physical process of 

 diffusion, which is generally considered to play a great part, is aided 

 by some other process, which may provisionally be termed secre- 

 tion, and which can move the gases even against the slope of pres- 

 sure. It is possible, too, that when the conditions are especially 

 unfavourable to diffusion when, for instance, the partial pressure 

 of carbon dioxide is artificially increased in the alveoli the cells 

 which line them are stimulated to increased activity, just as Bohr 

 has supposed that under the influence of the carbon monoxide used 

 in the observations of Haldane and Smith the absorption of oxygen 

 was greatly stimulated. 



Additional evidence in favour of the view that there is, besides 

 diffusion, an element of selective secretion in the exchange of gases 

 through the pulmonary membrane has been found by some writers 

 in the results of a study of the gases of the swim-bladder in fishes ; 

 and to the extent that this study has demonstrated the existence 

 of animal cells which actually secrete gases, it removes a presump- 

 tion against, if it does not establish a presumption in favour of, the 

 secretion theory of external respiration. These gases consist of 

 oxygen, nitrogen, and usually a small quantity of carbon dioxide, 



