316 VETERINARY PHYSIOLOGY 



air i.e. under a low pressure of carbon dioxide the gas is 

 given off ; but if the lime is in an atmosphere of C0 2 the gas 

 does not come off, but is taken up and the carbonate is formed. 



It will thus be seen that for every temperature there is a 

 certain pressure of the gas at which the solution or chemical 

 combination will neither give off nor take up more of the gas. 

 This may be determined by exposing the material in a series 

 of chambers to air containing different proportions of the gas, 

 and ascertaining by analyses of the air whether the gas has 

 been given off or taken up or has remained unaltered. 

 The presence of a moist membrane between the fluid and the 

 air makes no difference to these interchanges. 



For blood some form of serotonometer is used, an instru- 

 ment in which blood is allowed to trickle through a tube 

 filled with air of known composition till the tension of the 

 gases in the blood and air have become the same. The 

 tension is then determined by analysis of the proportions of 

 the gases in the air in the tube. 



We know that the pressure of gas in an atmosphere depends 

 upon the proportion present. Suppose an atmosphere contains 

 20 per cent, of oxygen, then the pressure, or partial pressure, 

 of the oxygen is got by multiplying the percentage amount 

 of the gas by 760 i.e. a whole atmosphere's pressure and 

 dividing by 100 



Pressure of oxygen = = 152 mm. 



The results obtained by means of the serotonometer in the 

 hands of different investigators have varied very widely. 



Another method of arriving at the tension of oxygen in the 

 blood has been devised by Haldane. It depends upon the 

 fact that if haemoglobin is exposed to a mixture of air with a 

 trace of carbon monoxide in it, the amount of carbon monoxide 

 taken up depends upon the oxygen tension the greater the 

 tension of oxygen the less CO will be taken up, and vice versa. 

 Therefore if a person be allowed to breathe an atmosphere 

 containing a known amount of CO, the amount of CO taken up 

 by the blood will indicate the oxygen tension in the blood. 



By such methods, although the results of different observers 

 vary so greatly, it appears that in blood going to the lungs 



