1074 PHYSIOLOGY 



determining the oxygen tension of the blood in the lungs founded on the use of carbon 

 monoxide. It has already been mentioned that carbon monoxide has the power of dis- 

 placing oxygen from oxyhaemoglobin to form a much more stable compound, carboxy- 

 hsemoglobin. If blood be shaken up with a mixture of oxygen and carbon monoxide, 

 the haemoglobin distributes itself between the two gases. In order, however, to get an 

 equal distribution, it is necessary to take a very small percentage of carbon monoxide, 

 owing to its greater avidity for haemoglobin. Thus, if haemoglobin solution be shaken 

 up with air containing -07 per cent, of CO, the result is a mixture of equal parts of oxy- 

 and carboxyhsemoglobin. The affinity of CO for haemoglobin would thus appear to be 



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about = 300 times the affinity of oxygen for haemoglobin. 



Carbon monoxide is not destroyed in the body, so that if a mixture containing 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 inspired air. The amount of haemoglobin which 

 is converted into carboxyhaemoglobin 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 the alveolar 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 experi- 

 ments, 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 differ- 

 ence 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 Teinvestigation 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 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 o the vagus nerves. Bohr, as the result of experiments by himself and some 



