READJUSTMENTS OF REGULATION 53 



the gas simply pass through them by ordinary diffu- 

 sion? This question has been debated ever since a 

 suggestion that they may play some active part was 

 made by Ludwig forty or fifty years ago. 



By means of an apparatus known as the aeroto- 

 nometer, Pfluger and his pupils compared the pressure 

 of C0 2 in the blood with that in alveolar air, and 

 found it to be about the same. The aerotonom- 

 eter was then improved by Bohr of Copenhagen, 

 an old pupil of Ludwig. His results seemed to show 

 that sometimes there is a lower pressure of C0 2 , and 

 a higher pressure of oxygen in the arterial blood than 

 in the lung air, in which case an active secretion of 

 oxygen inwards, and of C0 2 outwards, must be 

 assumed. Fredericq then got results in favour of the 

 simple diffusion theory. Last of all Krogh of Copen- 

 hagen improved the aerotonometer still further, and 

 obtained results which again favoured the diffusion 

 theory. 



Meanwhile Lorrain Smith and I attacked the prob- 

 lem by a new method, which was suggested to me by 

 the study of CO poisoning, and which eliminated cer- 

 tain sources of very serious error in the aerotonometer 

 method of measuring the arterial oxygen pressure. 

 When blood is saturated with a mixture containing 

 both oxygen and CO the haemoglobin combines partly 

 with CO and partly with oxygen in perfectly definite 

 proportions depending on the relative pressures of 

 the two gases, although in consequence of the far 

 greater affinity of CO for haemoglobin the pressure 

 of oxygen must be about 300 times greater than the 



