RESPIRATION 211 



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 invoked ; 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. But it gives us a minimum value, 

 below which it is not conceivable that the alveolar partial 

 pressure can lie, for we cannot imagine that any air in the 

 respiratory tract can be richer in carbon dioxide than that of 

 the alveoli. Now, Vierordt found with the deepest possible 

 expiration a little over 5 per cent, of carbon dioxide in the 

 expired air. From this it seems justifiable to conclude that 

 in man the partial pressure of carbon dioxide in the alveoli 

 may be at least one-twentieth of an atmosphere, or 38 mm. 

 of mercury. 



In animals, samples of the alveolar air have been drawn 

 off directly (Wolffberg) by means of Pfluger's 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., corre- 

 sponding to a partial pressure of about 29 mm. of mercury. 

 But this would be undoubtedly too high, owing to the im- 

 possibility of interchange with the external atmosphere, and 



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