1198 PHYSIOLOGY 



the result is a mixture of equal parts of oxy- and carboxyhaemoglobin. The 



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affinity of CO for haemoglobin would thus appear to be about ^ = 300 times 



the affinity of oxygen for haemoglobin. 



Carbon monoxide is not destroyed in the body, so that if a mixture con- 

 taining a small proportion of CO be breathed, this gas will be taken up until 

 a certain percentage of haemoglobin is converted into CO haemoglobin and the 

 tension of CO in the tissues and fluids of the body is equal to that of the in- 

 spired air. The amount of haemoglobin which is converted into carboxyhaemo- 

 globin will serve as a measure of the relative tensions of CO and oxygen in 

 the lungs. If the oxygen tension of arterial blood were the same as that of air, 

 we should expect that, with a given percentage of CO in the air breathed, the 

 final saturation with CO of the blood within the body would be the same as 

 the saturation of blood when shaken outside the body with air containing the 

 same percentage of CO as in the air breathed. It was found by Haldane, however, 

 that in all cases the percentage of CO haemoglobin formed was much less in the 

 body than outside the body. Thus in blood shaken up with air containing 

 20*9 per cent, oxygen and *045 per cent. CO, the amount of carbon monoxide 

 formed was 31 per cent, of the whole haemoglobin. When the same mixture was 

 inhaled for three or four hours by a man, the percentage of CO haemoglobin in 

 his blood rose only to 26 per cent., at which figure it remained stationary. This 

 would correspond to an oxygen tension of about 25 per cent, of an atmosphere, 

 whereas we have already seen that the oxygen tension in the alveoli cannot be 

 greater than 15 per cent. He therefore concluded that the epithelial cells of 

 the alveoli play an active part in the respiratory interchange, taking up the 

 oxygen on one side at a tension of 15 per cent, and piling it up on the other 

 until the pressure in the blood is much higher than that in the alveolar air. 

 Theoretically there is no reason to deny the possibility of such powers to the 

 pulmonary epithelium. We know that the secreting cells of the kidney take up 

 urea from the blood which contains only about -02 per cent, of this substance, 

 and excrete it into the renal tubule, into a medium containing about 2 per cent. ; 

 and if the data given by Haldane are correct we must ascribe an analogous 

 function to the pulmonary epithelium. These data, however, were obtained 

 by a colorimetric method working with very minute quantities of blood, and 

 lacked the support of control experiments. As a result of further experiments, 

 Haldane has modified his position so far as to allow that under normal 

 conditions the absorption of oxygen from the alveolar air takes place in 

 accordance with the difference of pressure, i.e. by a process of diffusion. He 

 is still of opinion that under abnormal conditions, when the oxygen tension 

 in the alveolar air is very low, there is an active absorption and transference 

 of oxygen to the blood on the part of the pulmonary epithelium. Why animals 

 should evolve a function which can only be brought into play on climbing 

 mountains seems difficult to understand, and it does not seem probable that 

 a reinvestigation of the tensions of oxygen in the blood under such conditions 

 by Krogh's method will lend any confirmation to Haldane's conclusions. 



An analogy has been drawn between the processes of gas interchange in the 

 lungs and that in the swim bladder of the fish. Bohr has shown that the gas 

 obtained by puncturing the bladder often contains a considerable excess of oxygen. 

 If the bladder be punctured and the fish then left in the water, the gas rapidly 

 reaccumulates, and it is found on tapping a second time that the percentage of 

 oxygen is largely increased, and may amount to between 60 and 80 per cent, of 

 the total gases. This reaccumulation of the gases does not take place if both 

 vagi are cut, and is therefore ascribed to a direct secretory activity on the part 

 of the epithelium lining the swim bladder under the influence of the vagus nerves. 



