THE CONTROL OF THE RESPIRATION 



355 



atmosphere in the space would show that the C0 2 percentage had been 

 raised. If the blood contained a lower tension than that corresponding 

 to the percentage of C0 2 in the space, some of the C0 2 would diffuse 

 into the blood, and its percentage in the atmosphere would be lowered. 

 By successively exposing blood to gas mixtures that contain slightly 

 different percentages of C0 2 , we should ultimately find one with which 

 the free C0 2 in the blood was in perfect equilibrium, and we should be 

 able to state that the tension of this gas in the blood was equal to a 

 certain percentage in the atmosphere surrounding the blood (see Fig. 

 121). 



Many forms of apparatus based on the above principle have been in- 

 vented for the examination of the tension of the gases in the blood. 

 The most accurate is that devised by Krogh, 18 the principle of which 



CO, 



IT 



Fig. 121. Diagram to show principle for measurement of the tension of 

 CO 2 tension of blood is supposed to be 5.75. 



blood. The 



differs slightly from that just described in that a bubble of air is 

 exposed to a relatively large quantity of blood, so that after a time 

 actual equilibrium of gas tension becomes established between the bub- 

 ble and the gases of the blood. This apparatus is shown in Figs. 122 

 and 123. It consists of a graduated tube of narrow bore sur- 

 rounded by a water jacket. To the upper end of the graduated tube 

 a small syringe is attached. The lower end of the graduated tube ex- 

 pands into a thistle-shaped bulb, closed below by a cork, through which 

 is inserted a tube (inflow tube) ending near the top of the bulb in a 

 fine opening and connected outside with an artery. An outflow tube is 

 also connected with the thistle-shaped bulb. 



At the beginning of the experiment the thistle-shaped bulb and the 

 graduated tube are filled with physiological saline. By means of the 

 syringe a small bubble of air is then introduced, so that it lies at the 



