1192 



PHYSIOLOGY 



Pfliiger's aerotonometer (Fig. 499) consists of two glass tubes, B and R, 

 contained in a vessel filled with water at the temperature of the body. The 

 upper ends of the tubes are connected by the tube a with the artery or vein 

 in which it is desired to estimate the tension of the blood-gases. If, for instance, 

 we wish to determine the tension of CO 2 in venous blood, where we expect the 

 tension to amount to about 4 per cent, of an atmosphere, one tube B is filled 

 with a gaseous mixture containing 3 per cent. C0 2 , and the other tube E with a 

 mixture containing 5 per cent. CO 2 . a is now connected with the distal end of 

 the jugular vein, or with the central end of the carotid artery, and blood is 

 allowed to flow in a thin stream down the walls of the tubes E and E, thus 

 presenting a large surface to the contained gases. The blood collects in the 



FIG. 499. Pfliiger's aerotonometer. 



lower narrower portions of the tubes, and runs out into the vessels b, b, whence 

 after defibrination it is returned at intervals into the veins of the animal. 



Bohr's aerotonometer was built on the plan of the Stromaiche devised by 

 Ludwig, and could be inserted in the course either of an artery or of a vein. In 

 using this instrument it is advisable to inject some substance like peptone or, 

 better, hirudin, in order to prevent coagulation of the blood. 



In all these instruments, however, the main difficulty is in obtain- 

 ing a sufficient surface of the blood exposed to the gaseous mixture. 

 The interchange of gases is thus very slow, and it is difficult to be certain 

 at any time that the blood and the gas with which it is in contact are 

 really in equilibrium. Krogh therefore adopted an ingenious device 

 of limiting the volume of air to a small bubble, the superficial area of 

 which is large in proportion to its bulk. This bubble, after it has been 

 in a stream of blood for some minutes, is transferred to a special 

 capillary tube in which its analysis can be carried out with a fair degree 

 of accuracy. 



