362 THE RESPIRATION 



by the actual tension under which the gas is evolved from the blood, is 

 less than the average tension of C0 2 in the alveolar air during the time 

 of a respiratory cycle. 



In the case of 2 the conditions are different. While the diluting 

 effect of the alveolar tidal air is marked in altering the amount of C0 2 

 given off during the different phases of a respiration, it can have little 

 influence on the amount of 2 taken up by the blood under normal con- 

 ditions. This is evident from a study of the dissociation curve of hemo- 

 globin (page 396), which shows that at tensions above 65 mm. Hg the 

 hemoglobin is practically saturated with 2 . Since the tension of 2 

 in the alveolar air under normal conditions is greater than 65 mm. 

 (95-100 mm.), the rate of absorption of 2 must be practically maximal 

 during the respiratory cycle that is, it will not change at different 

 phases of it. 



While the relationship of the alveolar gases is continually changing 

 at different stages of the respiratory cycle, their mean relationship for 

 periods including several respirations or for complete respirations is 

 more or less constant, being controlled by the type of the metabolism, 

 and mathematically expressed by the respiratory quotient (page 582). 

 The average relative percentages of the two gases in the alveolar air 

 must therefore be the same as in the tidal air. In the alveolar air col- 

 lected by the Haldane method, however, the above factors cause the 

 respiratory quotient to be less than that in the tidal air. 



These points have been insisted upon because much of the knowledge 

 of the gaseous exchange between the blood and the air in the lungs, as 

 well as the control of respiration, has been built upon data obtained by 

 the Haldane-Priestly method, and in considering this work, which we 

 shall do in subsequent pages, it is advisable that we be aware of the 

 limitations of the method employed. The method has been an invaluable 

 one for opening up a hitherto entirely unexplored field of research, but 

 now, the pioneer work having been done, we must employ methods 

 which will enable us to explore more exactly. 



An Accurate Standard Method for Normal Subjects. The most accu- 

 rate method, and one free from many of the theoretical errors present in 

 the others, depends on the relationship found to exist between the dilut- 

 ing effect of the air in the dead space (see page 319) and the known per- 

 centage composition of the alveolar air in expirations which are of vary- 

 ing depths but of equal and normal duration and which follow normal 

 inspirations (R. G. Pearce). 



In this method the subject is made to breathe through valves, which automatically 

 separate the inspired from the expired air. The expired air is led into a tube con- 

 nected with two spirometers by two three-way stopcocks. The spirometers are of the 

 Gad-Krogh type, one being capable of holding ten liters, and the other one and a half. 



