112 EXPERIMENTAL PHYSIOLOGY. 



Because of the free diffusibility of CO 2 through the pul- 

 monary endothelium, the same tension must exist in the blood 

 as in alveolar air. If the alveolar air be collected with the least 

 possible disturbance in breathing, this equilibrium will be 

 between the arterial blood leaving the lungs but if the breath 

 be held it will be between the venous blood as it comes to 

 them. The former value, i.e., the arterial tension of CO^, is 

 of very great importance partly because it regulates the H-ion 

 concentration of the blood going to the respiratory and other 



H 2 CO 3 

 centres (through the equation r ) an( ^ P ar tly because of 



its specific action on these centres. The most practical method 

 for collecting alveolar air without serious disturbance of breath- 

 ing is that of Haldane. 



Experiment 49a. Haldane's method for collecting alveolar 

 air. The apparatus consists of a piece of wide bore rvibber 

 tubing about 1 metre long provided at one end with a mouth 

 piece, consisting of a glass or brass tube of similar bore to the 

 rubber tubing, and somewhat flattened at the free end, with a 

 short piece of narrow-bore tubing attached at right angles. 

 After dipping in antiseptic solution place the mouth piece in 

 the mouth of the observed person and direct him to close the 

 lips tightly round it, and to continue breathing, by inspiring 

 through the nose and expiring through the tube for a minute or 

 two so that the person may become accustomed to the pro- 

 cedure. Meanwhile attach a clean all-glass 10 c.c. syringe with 

 the piston lightly smeared with vaseline (if there is too much 

 vaseline it will absorb an appreciable amount of CO 2 ) to the 

 side tube by means of a short piece of rubber tubing. Now 

 instruct the person to make a forced expiration (really to blow 

 out) through the tube for a period of time equal to that of an 

 ordinary expiration and to close the opening of the tube in the 

 mouth with the tongue when he has completed the expiration' 

 Immediately he has done this, withdraw the piston of the 

 syringe (not too hurriedly so as to avoid breaking the syringe) 

 and when a little over 10 c.c. of air has been collected ins.truct 

 the person to continue breathing through the tube as before. 



