360 THE RESPIRATION 



higher than that in the alveolar air. The earliest method for measuring 

 it consisted in passing a lung catheter into the right bronchus and then 

 blocking the passage above the open end of the catheter by inflating a 

 rubber collar or ampulla. The renewal of air in the right lung is thereby 

 prevented, and a sample of the stagnant air can be removed and analyzed. 

 In such a case, however, the blood will have circulated several times 

 round the body, and with only one lung operating the risk is incurred 

 that more C0 2 is being discharged into the blocked lung than cor- 

 responds to the tension of C0 2 of venous blood under normal conditions. 



Much more practical methods are those of Haldane, Yandell Hender- 

 son and R. G. Pearce, which are much the same in principle. In Pearce's 

 method, the person first of all inspires from a gas meter containing a 

 gaseous mixture with about 10 per cent of C0 2 . Immediately after fill- 

 ing the lungs, he makes a rapid forced expiration into a tube provided 

 with a valve having four openings. This valve is turned through a 

 complete circuit during the expiration, so that four fractions of the ex- 

 pired air can be collected in rubber bags connected with side tubes 

 opening opposite the four openings in the valve. The first fraction will 

 contain a little less than 10 per cent C0 2 , the second distinctly less, 

 while the fourth will contain the same as the third, indicating that equi- 

 librium between the C0 2 of the alveolar air and the blood must have been 

 attained. This figure therefore gives us the tension of C0 2 in the venous 

 blood of the lungs. In Henderson's method the rebreathing is per- 

 formed into gas receivers containing 6 per cent C0 2 . 



These results then indicate that the whole process by which C0 2 is 

 exchanged in the lungs is dependent on the law of gas diffusion ; the gas 

 diffuses from a place of higher to a place of lower pressure, and does 

 so until equilibrium is attained. 



The tension of 2 in venous blood may be determined by the applica- 

 tion of similar principles. After a deep expiration nitrogen is inspired 

 from a rubber bag for 3-5 breaths (10-20 sees.) and a sample of alveolar 

 air is immediately taken. The percentage of 2 in this alveolar air will 

 be found to be the same whether it be taken after 3 or 5 breaths, show- 

 ing that an equilibrium with the tension of this gas in the blood must 

 have been reached. If taken earlier, the 2 will be too high, and if taken 

 later it will be too low (because the blood will have circulated twice 

 round the body). This 2 percentage corresponds with the partial pres- 

 sure (tension) of 2 in the mixed venous blood but it is somewhat lower 

 than the latter because under the conditions of the experiment the 

 C0 2 in the alveolar air is only that of arterial blood (40 mm.) (Bar- 

 croft, etc. 84 ). 



