RESPIRATION 257 



The pressure of carbon dioxide in arterial blood we may take 

 at 10 to 40 mm., in venous blood at 30 to 50 mm., according to 

 the results of different observers. 



Whenever the venous blood has to pass through a region in 

 which the pressure of carbon dioxide is higher than its own, 

 carbon dioxide will enter it. When it enters a region in which the 

 carbon dioxide pressure is kept lower than in itself, the carbon 

 dioxide compounds formed in its passage through the tissues will be 

 dissociated, and it will begin to lose carbon dioxide by diffusion. 

 If the pressure of oxygen in this region is at the same time higher 

 than in the venous blood, some of it will be taken up. And to 

 bring about these results no peculiar ' vital ' force need be in- 

 voked ; ordinary physical processes will, under the assumed 

 conditions, be alone required. 



Now, we know that in the lungs carbon dioxide is given off 

 from the blood, and oxygen taken up by it. We have, therefore, 

 to inquire what the partial pressures of these gases are in the 

 alveoli, and whether they are so related to the corresponding 

 partial pressures in the blood that a simple process of dissociation 

 and diffusion will be sufficient to explain pulmonary respiration. 



The percentage of carbon dioxide in expired air cannot tell us 

 the pressure of that gas in the alveoli, for the air in the upper 

 part of the respiratory tract is necessarily expelled along with 

 the alveolar air, and dilutes the carbon dioxide in it. But the 

 mean of the carbon dioxide percentages in samples taken from 

 the last portions of the air of two deep expirations, one following an 

 ordinary inspiration and the other following an ordinary expiration, 

 is the mean percentage in the alveoli. This quantity, while, as already 

 remarked (p. 242), very constant in a given individual, varies in 

 different men from 4* 6 to 6'2 (mean 5 - 5) per cent, of the dry alveolar 

 air. In women and in children of both sexes it is less than in 

 men. From this we conclude that in men the partial pressure of 

 carbon dioxide in the alveoli may be at least one-eighteenth of 

 an atmosphere, or 42 mm. of mercury (Fitzgerald and Haldane). 



In animals, samples of the alveolar air have been drawn off 

 directly by means of a pulmonary catheter. This consists of two 

 tubes, one within the other. The inner tube, which is a fine 

 elastic catheter, projects free from the other for a little distance 

 at its lower end. The outer tube terminates in an indiarubber 

 ball, which can be inflated so as to block the bronchus into which 

 it is passed, and cut off the corresponding portion of the lung 

 from communication with the outer air. A sample of the air 

 below the block can be drawn off through the inner tube. In 

 this way the proportion of carbon dioxide in the alveoli of the dog 

 was found to be only about 3*8 per cent., corresponding to a 

 partial pressure of about 29 mm. of mercury. 



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