THE CHEMISTRY OF RESPIRATION 



491 



usually accomplished in a perfectly direct way with the help of an instrument 

 known as an aerotonometer. The apparatus, devised by Pfliiger, consists of two 

 glass tubes which are placed in a receptacle containing water at 37° C.^ One of 

 these is filled with a gaseous mixture having a greater and the other with a gaseous 

 mixture having a lesser partial pressure than is expected to be found in the blood 

 under examination. Thus, if it is our intention to determine the tension of the 

 CO2 in venous blood, which may be estimated at about 4 per cent., one of these 

 tubes is filled with a mixture containing 3 per cent. CO2, and the other with a 

 mixture containing 5 per cent. CO2. On permitting the blood to run in a thin 



Fig. 253. — A, Krogh's Microtonometeb. B, Upper Part of Microtonometer 

 Showing Capillary Tube into Which the Bubble is Returned for Measurement and 

 Analysis. 



layer down the walls of these tubes, it yields CO 2 to one mixture and abstracts it 

 from the other. The proportion of CO2 found in the mixtures at the end of the 

 experiment, forms the basis of the calculation of the partial pressure of thq CO2 

 in the blood, because this value corresponds to the partial pressure which would 

 have to prevail in the tubes in order that the blood be able to traverse them 

 without suffering a change in its CO 2 content. 



The aerotonometer of Bohr^ embodies the principle of the stromuhr and 

 permits the blood to reenter the blood-vessel after it has been temporarily diverted 

 into the gas chamber. On this account, these determinations may be continued 

 for a much longer period of time, allowing a thorough equilibrium to be established. 

 Krogh^ uses a small bubble of air which is brought into contact with a correspond- 



1 Modified by Fredericq, Zentralbl. fiir Physiol., viii, 1894, 34. 



2 Skand. Archiv fiir Physiol., ii, 1900, 236. 



3 Ibid., XX, 1908, 279. 



