680 



PHYSIOLOGY OF RESPIRATION. 



gen is held is measured by the pressure of the gas in the sur- 

 rounding atmosphere at which the 

 compound neither takes up nor 

 gives off oxygen. If, therefore, it 

 is necessary to determine the ten- 

 sion of any gas held in solution 

 or in dissociable combination it is 

 sufficient to determine the per- 

 centage of that gas in the sur- 

 rounding atmosphere and thus 

 ascertain the partial pressure that 

 it exerts. If the atmosphere con- 

 tains 5 per cent, of a given gas 

 the partial pressure exerted by it, 

 leaving out of account the aqueous 

 tension, is equal to 38 mms. Hg. 

 (760 X 0.05) , and this figure express- 

 es the tension under which the gas 

 is held in solution or combination in 

 a liquid exposed to such an atmos- 

 phere. As regards the tension of 

 the gases in arterial and venous 

 blood, this procedure is, of course, 

 not possible, since the blood is 

 surrounded, not by an atmosphere 

 whose composition can be anal- 

 yzed, but by the liquids of the 

 body, the lymph and cell juices. 

 To determine the tension of the 

 gases in the blood it is necessary 

 to remove the blood from the 

 vessels and bring it into contact 

 with an atmosphere containing a 

 known quantity of O, CO 2 , or N, 

 according to the gas to be measured. By trial an atmosphere can 

 be obtained in which this gas is contained in amounts such that there 

 is no marked increase or decrease in quantity after standing in diffu- 

 sion relations with the blood. The percentage of the gas in the at- 

 mosphere chosen will measure the tension of that gas in the blood. 

 An instrument which has been much used for such determinations is 

 represented diagrammatically in Fig. 274. It is known as a tonom- 

 eter or aerotonometer (Pfliiger). It consists of a tube (A) which 

 can be connected through 6 directly with the blood-vessels. This 

 tube A is surrounded by a jacket (C) containing warm water, so 



274. Diagram to show the 

 principle of the aerotonometer: A, The 

 tube containing a known mixture of 

 gases, O, CO ? , N; C, the outside jacket 

 for maintaining a constant body tem- 

 perature. When stopcock 6 is open 

 the blood trickles down the sides of A 

 and enters into diffusion relations with 

 the contained gases. After equilibrium 

 is reached the stopc'ock t; is closed and 

 a is opened. By means of the mer- 

 cury bulb the gases can then be forced 

 out of A into a suitable receiver for 

 analysis. 



