CHANGES IN AIR AND BLOOD IN RESPIRATION. 



675 



Hg; that of the nitrogen is 608 mms. Hg. If the same water is 

 exposed to pure oxygen the tension of the oxygen in solution is 

 equal to 760 mms. Hg, while that of the nitrogen sinks to zero if 

 the gas that is given off from the water is removed. With com- 

 pounds such as oxyhemoglobin the tension under which the oxy- 

 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 

 is equal to 38 mms. Hg. (760 X 

 0.05), and this figure expresses 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 . ?ig. 2J4 Diagram to show the 

 ? j ,1 i i im principle of the aerotonometer : A, The 

 DOdy, the lymph and Cell juices. tube containing a known mixture of 



To determine the tension of the for e main'tainuig a constant body a tem- 



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, C0 2 , or N, 

 according to the gas to be meas- 

 ured. 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 diffusion relations with the 

 blood. The percentage of the gas in the atmosphere chosen will 

 measure the tension of that gas in the blood. An instrument 

 which has been much used for such determinations is represented 



mamtainuig a constant bodj 

 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 stopcock b 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. 



