338 



THE RESPIRATION 



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 w r hich 



co t 



1 



f.S e,t 

 C0 a 5.1 T 



co t 



IT 



Eig. 121. Diagram to show principle for measurement of the tension of CQz in blood. The 

 CC>2 tension of blood is supposed to be 5.75. 



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 physiologic saline. By means of the 

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



