630 PRINCIPLES OF GENERAL PHYSIOLOGY 



Douglas, Haldane, Henderson, and Schneider which seems a little strange, 

 although it may have no significance. Notwithstanding that the arterial oxygen 

 tension was always higher than that given for the alveolar air, it was never as 

 high as that of the atmosphere at the time, although occasionally not much below 

 it. Why should the secretory power fail just at this level and not raise the 

 oxygen tension above that of the atmosphere ? Is it possible that the blood had 

 como into equilibrium with oxygen tension somewhere which was not given 

 correctly by the measurement of that of the alveolar air? 



Might it not also be possible that the carbon monoxide method gives different 

 values when the haemoglobin content of the blood is increased, as in the case of 

 acclimatisation to high altitudes? Hasselbalch (1912) shows that the hydrogen 

 ion concentration is increased under these circumstances. 



This question of secretion by the lungs is instructive from the point of view of " vitalism." 

 When first proposed, it was held to apply to the ordinary state of affairs ; but, as improve- 

 ments were made in experimental methods, the absorption was shown to follow physical 

 laws ; it was then held to apply to cases of muscular exercise, and now only to acclimatisation 

 to high altitudes. One might venture to say that the more accurate the methods of 

 investigation, the better is it found that chemical and physical laws are capable of explaining 

 physiological phenomena. 



THE REGULATION OF RESPIRATION 



By Hydrogen Ion Concentration of the Blood. The renewal of the air with 

 which the blood interchanges its gaseous constituents is effected by muscular 

 movements, and it is plain that the rate of change of the air in the lungs needs to 

 be varied in order to provide for the different rates at which oxygen is consumed 

 and carbon dioxide evolved in states of rest and of activity. We have to inquire 

 how this regulation is effected. 



The co-ordination of the muscular movements required is effected by the 

 " respiratory centre " in the bulb, which sends out periodic discharges to the motor 

 neurones of the spinal segments in which the muscles concerned are represented. 

 Like other nerve centres, this centre is capable of being influenced by afferent 

 impulses, especially from the lungs themselves. The function of these will be 

 seen later. 



It is the great merit of Haldane and Priestley (1905) to have shown that the 

 regulation of respiration, meaning by that the amount of ventilation per unit of time, 

 or the total volume of air sent in and out of the lungs, is effected by the carbon 

 dioxide tension of the arterial blood, which is the same as that of the alveolar air 

 of the lungs. Later work showed that the hydrogen ion concentration, due to the 

 dissolved carbon dioxide, is the actual exciting agent. The cells or synapses of 

 the respiratory centre must, therefore, be very sensitive to changes in the 

 concentration of the hydrogen ions of the blood. 



Now, the venous blood from the organs does not pass directly to the centre, 

 but only after having interchanged with the alveolar air. The carbonic acid 

 tension of the arterial blood is then the determining factor of the ventilation. It 

 is thus of some importance to know how this value is related to that of the 

 alveolar air, and this again to that of the venous blood. Bohr thought it necessary 

 to assume, along with oxygen secretion, an active excretion of carbon dioxide on 

 the part of the pulmonary epithelium. Krogh's experiments, already mentioned, 

 showed the carbon dioxide tension of the arterial blood to be equal to that of the 

 alveolar air, not less, as it would be if actively excreted. He points out that 

 the remarkable sensibility of the respiratory centre to a slight increase of the 

 carbon dioxide tension of the alveolar air would be upset by interference with 

 the relation between that of the alveolar air and that of the arterial blood, such 

 as would result from an excretory process. Haldane and Priestley, in fact, showed 

 that a rise of the carbon dioxide tension in the lung alveoli of only 1'6 mm. of 

 mercury, or of 0'22 per cent, of its content in carbon dioxide, increases the 

 ventilation of the lungs to double its previous value. If the carbon dioxide 

 tension of the venous blood rises by a very small amount, that of the alveolar air 

 will also rise by diffusion, so tTiat the arterial blood leaving the lungs will have 



