628 PRINCIPLES OF GENERAL PHYSIOLOGY 



What we have to do is to determine the oxygen and carbon dioxide tensions 

 in arterial blood, and compare them with the alveolar tensions of the two gases. 

 The methods used to do this are the carbon monoxide method of Haldane and 

 Douglas, already referred to, and the aerotonometer methods. The latter consist 

 in exposing the blood to a limited volume of gas, of a composition as nearly as 

 possible the same as regards the tensions of its components as that of the blood. 

 After equilibrium has been attained, the composition of the gas phase is estimated 

 by the usual methods of gas analysis. It is doubtful whether the earlier experi- 

 ments were trustworthy, since the volume of the gas was too large. Those of 

 Krogh (1910) are free from this objection. He 'devised a method by which a 

 small bubble of gas can be analysed with the greatest accuracy. This bubble of 

 gas was exposed to the current of arterial blood, being kept in constant motion 

 by the current. After a time it was transferred to the measuring tube, and the 

 carbon dioxide and oxygen contained in it determined. The result was that the 

 tension of oxygen in arterial blood, under the conditions of the experiments, was. 

 always lower than that of the alveolar air. Hence, so far, there is no difficulty 

 in the diffusion theory. The tension of carbon dioxide was found practically 

 identical with that of the alveolar air, but never less. The actual value of the 

 carbon dioxide tension will be referred to again later. 



At this point we must consider the view taken by Bohr, who believed that 

 his experiments showed that the alveolar epithelium has the power of actively 

 secreting oxygen in the direction of the blood, so that the tension of oxygen in 

 the arterial blood may be higher than in the alveolar air. Certain physiologists, 

 Haldane, Douglas, and Barcroft, to mention three only, still hold this view in a 

 modified way. While admitting that the evidence is against the secretion of 

 oxygen under ordinary conditions of rest and even of temporary want of oxygen, 

 as in muscular exercise, they hold (see especially Douglas, Haldane, Henderson, 

 and Schneider, 1913, pp. 204 and 205) that, during the process of acclimatisation 

 to a high altitude, with its low oxygen tension, the lung epithelium develops 

 the power of secreting oxygen. The table given on p. 197 of the paper named 

 gives a number of data, and it will be seen that the oxygen pressures in arterial 

 blood, as determined by the carbon monoxide method, are considerably higher than 

 in the alveolar air in all the cases which had become acclimatised. 



It is a difficult matter to understand how such a function should have been 

 formed in the course of evolution to meet a need very rarely arising. We must 

 remember that it is only supposed to show itself after exposure to want of oxygen 

 for a considerable time. We may also consider briefly some further objections 

 brought by Krogh against the view. In the first place, Krogh points out that, 

 owing to the form of the dissociation curve of oxyh^moglobin, the haemoglobin is 

 nearly saturated at the ordinary alveolar tension of oxygen, so that, in order to 

 increase the oxygen percentage by 0'4 per cent, only, an increase of tension of 30 

 per cent, would be necessary. Of course, this is not so serious an objection when 

 the alveolar oxygen tension is as low as that on the top of Pike's Peak, namely, 

 about 60 mm. ; but, even at this pressure, the haemoglobin is 86 per cent, saturated. 

 In any case, it seems a poor result for a new and special mechanism to be formed. 

 As far as concerns the actual work required, A. V. Hill (1913, 3) shows that what 

 is actually necessary to raise the oxygen tension from that of the alveoli to that of 

 the arterial blood in Douglas, Haldane, Henderson, and Schneider's experiments is 

 a very small fraction of that done by the organism as a whole. The value is given 

 by the expression we have frequently made use of : 



P\ 



where p 2 is the higher pressure and, of course, the integral has to be taken along 

 the particular limits of the dissociation curve corresponding to the respective 

 tensions. In man, the amount of energy per minute works out at about one gram 

 calorie. This might be done by epithelial cells of 0'5 p. in thickness, if their 

 efficiency were 20 per cent., that is, no greater than that of the body as a whole. 

 Another criticism made by K*rogh is that the histological structure of the pulmonary 



