EXPERIMENTAL PHYSIOLOGY 



which represents the percentage of this gas which is in equi- 

 librium with its tension in the pulmonary venous blood. 



A rough approximate idea of the venous tension of CO 2 can 

 be obtained by determination of its percentage in the alveolar 

 air at the breaking point, which means the point beyond which 



. J:he breath can no longer be held. 



Experiment 49b.~ After a normal inspiration hold the breath 

 as long as possible, then make a forced expiration through a 

 Haldane tube and collect a sample of the air for analysis of 

 COj (two if possible). Repeat this experiment after taking a 

 deep inspiration immediately before holding the breath. Keep 

 a precise record of the time during ^which the breath was held, 

 in both observations. Count the pulse and examine for cya- 

 nosis during the time the breath is held. The percentage of 

 CO 2 in alveolar air at the breaking point is considerably above 

 the CO 2 tension of the normal venous blood because the blood 

 has completed the circuit of the circulation several times during 

 the time the breath was held. Each time the blood returns 

 to the lungs it of course carries more and more CO 2 which 

 raises the tension of this gas. 



The breaking point experiment really tells us the tension 

 of CO 2 in arterial blood beyond which the activities of the 

 respiratory centre can no longer be inhibited by voluntary 

 effort. This tension will not always be the same because it 

 will depend on whether or not other chemical changes are 

 occurring in the blood that are capable of affecting the excita- 

 bility of the centre. One such change is the O 2 tension of the 

 blood. When this becomes depressed below a certain level the 

 respiratory centre is stimulated and it is consequently to be ex- 

 pected that part of the stimulus at the breaking point is due 

 to this cause. That this is the case can be shown by repeating 

 the breaking point experiment after filling the lungs with 

 oxygen. 



Experiment 49c. Take a deep inspiration from a rubber bag or 

 spirometer containing oxygen, then hold the breath to the 

 breaking point and analyse a specimen of alveolar air for CO 2 

 (and for O 2 if possible). Keep a record of the time during 

 which the breath was held. 



